perf: partially replace .concat with .push

packages/babel-cli/src/babel-external-helpers.ts

@@ -10,7 +10,11 @@ function collect(
 
   const values = value.split(",");
 
-  return previousValue ? previousValue.concat(values) : values;
+  if (previousValue) {
+    previousValue.push(...values);
+    return previousValue;
+  }
+  return values;
 }
 
 commander.option(

packages/babel-cli/src/babel/options.ts

@@ -192,7 +192,8 @@ export default function parseArgv(args: Array<string>): CmdOptions | null {
   let filenames = commander.args.reduce(function (globbed, input) {
     let files = glob.sync(input);
     if (!files.length) files = [input];
-    return globbed.concat(files);
+    globbed.push(...files);
+    return globbed;
   }, []);
 
   filenames = Array.from(new Set(filenames));
@@ -353,5 +354,9 @@ function collect(
 
   const values = value.split(",");
 
-  return previousValue ? previousValue.concat(values) : values;
+  if (previousValue) {
+    previousValue.push(...values);
+    return previousValue;
+  }
+  return values;
 }

packages/babel-helper-define-map/src/index.js

@@ -42,8 +42,8 @@ export function push(
   if (node.decorators) {
     const decorators = (map.decorators =
       map.decorators || t.arrayExpression([]));
-    decorators.elements = decorators.elements.concat(
-      node.decorators.map(dec => dec.expression).reverse(),
+    decorators.elements.push(
+      ...node.decorators.map(dec => dec.expression).reverse(),
     );
   }
 

packages/babel-helpers/src/helpers.js

@@ -316,7 +316,7 @@ helpers.extends = helper("7.0.0-beta.0")`
   }
 `;
 
-// This old helper can be removed in babel v8
+// TODO(babel-8): This old helper can be removed in babel v8
 helpers.objectSpread = helper("7.0.0-beta.0")`
   import defineProperty from "defineProperty";
 
@@ -325,7 +325,7 @@ helpers.objectSpread = helper("7.0.0-beta.0")`
       var source = (arguments[i] != null) ? Object(arguments[i]) : {};
       var ownKeys = Object.keys(source);
       if (typeof Object.getOwnPropertySymbols === 'function') {
-        ownKeys = ownKeys.concat(Object.getOwnPropertySymbols(source).filter(function(sym) {
+        ownKeys.push.apply(ownKeys, Object.getOwnPropertySymbols(source).filter(function(sym) {
           return Object.getOwnPropertyDescriptor(source, sym).enumerable;
         }));
       }

packages/babel-node/src/_babel-node.js

@@ -21,7 +21,11 @@ function collect(value, previousValue): Array<string> {
 
   const values = value.split(",");
 
-  return previousValue ? previousValue.concat(values) : values;
+  if (previousValue) {
+    previousValue.push(...values);
+    return previousValue;
+  }
+  return values;
 }
 
 program.option("-e, --eval [script]", "Evaluate script");
@@ -197,7 +201,7 @@ if (program.eval || program.print) {
     }
 
     // add back on node and concat the sliced args
-    process.argv = ["node"].concat(args);
+    process.argv = ["node", ...args];
     process.execArgv.push(fileURLToPath(import.meta.url));
 
     Module.runMain();

packages/babel-node/src/babel-node.js

@@ -8,7 +8,7 @@ import path from "path";
 import child_process from "child_process";
 import { fileURLToPath } from "url";
 
-let args = [
+const args = [
   path.join(path.dirname(fileURLToPath(import.meta.url)), "_babel-node"),
 ];
 
@@ -69,7 +69,7 @@ getV8Flags(async function (err, v8Flags) {
 
   // append arguments passed after --
   if (argSeparator > -1) {
-    args = args.concat(userArgs);
+    args.push(...userArgs);
   }
 
   try {

packages/babel-parser/src/plugins/flow/index.js

@@ -1961,7 +1961,8 @@ export default (superClass: Class<Parser>): Class<Parser> =>
 
         if (failed && valid.length === 1) {
           this.state = state;
-          this.state.noArrowAt = noArrowAt.concat(valid[0].start);
+          noArrowAt.push(valid[0].start);
+          this.state.noArrowAt = noArrowAt;
           ({ consequent, failed } = this.tryParseConditionalConsequent());
         }
       }

packages/babel-plugin-proposal-async-generator-functions/src/index.js

@@ -47,7 +47,7 @@ export default declare(api => {
       }
 
       // push the rest of the original loop body onto our new body
-      block.body = block.body.concat(node.body.body);
+      block.body.push(...node.body.body);
 
       t.inherits(loop, node);
       t.inherits(loop.body, node.body);

packages/babel-plugin-proposal-decorators/src/transformer-legacy.js

@@ -157,7 +157,7 @@ function applyTargetDecorators(path, state, decoratedProps) {
 
       WARNING_CALLS.add(node.value);
 
-      acc = acc.concat([
+      acc.push(
         t.assignmentExpression(
           "=",
           t.cloneNode(descriptor),
@@ -184,9 +184,9 @@ function applyTargetDecorators(path, state, decoratedProps) {
             ]),
           ]),
         ),
-      ]);
+      );
     } else {
-      acc = acc.concat(
+      acc.push(
         t.callExpression(state.addHelper("applyDecoratedDescriptor"), [
           t.cloneNode(target),
           t.cloneNode(property),

packages/babel-plugin-transform-classes/src/transformClass.ts

@@ -136,9 +136,8 @@ export default function transformClass(
 
     if (classState.userConstructor) {
       const { constructorBody, userConstructor, construct } = classState;
-      constructorBody.body = constructorBody.body.concat(
-        userConstructor.body.body,
-      );
+
+      constructorBody.body.push(...userConstructor.body.body);
       t.inherits(construct, userConstructor);
       t.inherits(constructorBody, userConstructor.body);
     }
@@ -695,7 +694,7 @@ export default function transformClass(
 
     extractDynamicKeys();
 
-    let { body } = classState;
+    const { body } = classState;
     const { closureParams, closureArgs } = setupClosureParamsArgs();
 
     buildBody();
@@ -712,8 +711,10 @@ export default function transformClass(
       );
     }
 
-    body = body.concat(
-      classState.staticPropBody.map(fn => fn(t.cloneNode(classState.classRef))),
+    body.push(
+      ...classState.staticPropBody.map(fn =>
+        fn(t.cloneNode(classState.classRef)),
+      ),
     );
 
     const isStrict = path.isInStrictMode();

packages/babel-plugin-transform-for-of/src/no-helper-implementation.js

@@ -24,7 +24,7 @@ export default function transformWithoutHelper(loose, path, state) {
   }
 
   // push the rest of the original loop body onto our new body
-  block.body = block.body.concat(node.body.body);
+  block.body.push(...node.body.body);
 
   t.inherits(loop, node);
   t.inherits(loop.body, node.body);

packages/babel-plugin-transform-modules-systemjs/src/index.js

@@ -295,7 +295,7 @@ export default declare((api, options) => {
           const exportMap = Object.create(null);
           const modules = [];
 
-          let beforeBody = [];
+          const beforeBody = [];
           const setters = [];
           const sources = [];
           const variableIds = [];
@@ -475,7 +475,7 @@ export default declare((api, options) => {
           }
 
           modules.forEach(function (specifiers) {
-            let setterBody = [];
+            const setterBody = [];
             const target = scope.generateUid(specifiers.key);
 
             for (let specifier of specifiers.imports) {
@@ -540,8 +540,8 @@ export default declare((api, options) => {
                 }
               }
 
-              setterBody = setterBody.concat(
-                constructExportCall(
+              setterBody.push(
+                ...constructExportCall(
                   path,
                   t.identifier(exportIdent),
                   exportNames,
@@ -589,8 +589,8 @@ export default declare((api, options) => {
           }
 
           if (exportNames.length) {
-            beforeBody = beforeBody.concat(
-              constructExportCall(
+            beforeBody.push(
+              ...constructExportCall(
                 path,
                 t.identifier(exportIdent),
                 exportNames,

packages/babel-plugin-transform-parameters/src/rest.js

@@ -305,9 +305,7 @@ export default function convertFunctionRest(path) {
     return true;
   }
 
-  state.references = state.references.concat(
-    state.candidates.map(({ path }) => path),
-  );
+  state.references.push(...state.candidates.map(({ path }) => path));
 
   const start = t.numericLiteral(paramsCount);
   const key = scope.generateUidIdentifier("key");

packages/babel-traverse/src/path/family.ts

@@ -46,7 +46,9 @@ function addCompletionRecords(
   records: Completion[],
   context: CompletionContext,
 ): Completion[] {
-  if (path) return records.concat(_getCompletionRecords(path, context));
+  if (path) {
+    records.push(..._getCompletionRecords(path, context));
+  }
   return records;
 }
 
@@ -73,9 +75,9 @@ function completionRecordForSwitch(
     if (normalCompletions.length) {
       lastNormalCompletions = normalCompletions;
     }
-    records = records.concat(breakCompletions);
+    records.push(...breakCompletions);
   }
-  records = records.concat(lastNormalCompletions);
+  records.push(...lastNormalCompletions);
   return records;
 }
 
@@ -117,7 +119,7 @@ function getStatementListCompletion(
   paths: NodePath[],
   context: CompletionContext,
 ): Completion[] {
-  let completions = [];
+  const completions = [];
   if (context.canHaveBreak) {
     let lastNormalCompletions = [];
     for (let i = 0; i < paths.length; i++) {
@@ -155,11 +157,11 @@ function getStatementListCompletion(
           // When we have seen normal completions from the last statement
           // it is safe to stop populating break and mark normal completions as break
           normalCompletionToBreak(lastNormalCompletions);
-          completions = completions.concat(lastNormalCompletions);
+          completions.push(...lastNormalCompletions);
           // Declarations have empty completion record, however they can not be nested
           // directly in return statement, i.e. `return (var a = 1)` is invalid.
           if (lastNormalCompletions.some(c => c.path.isDeclaration())) {
-            completions = completions.concat(statementCompletions);
+            completions.push(...statementCompletions);
             replaceBreakStatementInBreakCompletion(
               statementCompletions,
               /* reachable */ true,
@@ -170,7 +172,7 @@ function getStatementListCompletion(
             /* reachable */ false,
           );
         } else {
-          completions = completions.concat(statementCompletions);
+          completions.push(...statementCompletions);
           if (!context.shouldPopulateBreak) {
             replaceBreakStatementInBreakCompletion(
               statementCompletions,
@@ -181,14 +183,18 @@ function getStatementListCompletion(
         break;
       }
       if (i === paths.length - 1) {
-        completions = completions.concat(statementCompletions);
+        completions.push(...statementCompletions);
       } else {
-        completions = completions.concat(
-          statementCompletions.filter(c => c.type === BREAK_COMPLETION),
-        );
-        lastNormalCompletions = statementCompletions.filter(
-          c => c.type === NORMAL_COMPLETION,
-        );
+        lastNormalCompletions = [];
+        for (let i = 0; i < statementCompletions.length; i++) {
+          const c = statementCompletions[i];
+          if (c.type === BREAK_COMPLETION) {
+            completions.push(c);
+          }
+          if (c.type === NORMAL_COMPLETION) {
+            lastNormalCompletions.push(c);
+          }
+        }
       }
     }
   } else if (paths.length) {
@@ -202,7 +208,7 @@ function getStatementListCompletion(
         (pathCompletions.length === 1 &&
           !pathCompletions[0].path.isVariableDeclaration())
       ) {
-        completions = completions.concat(pathCompletions);
+        completions.push(...pathCompletions);
         break;
       }
     }
@@ -225,29 +231,25 @@ function _getCompletionRecords(
     path.isLabeledStatement()
   ) {
     // @ts-expect-error(flow->ts): todo
-    records = addCompletionRecords(path.get("body"), records, context);
+    return addCompletionRecords(path.get("body"), records, context);
   } else if (path.isProgram() || path.isBlockStatement()) {
-    records = records.concat(
-      // @ts-expect-error(flow->ts): todo
-      getStatementListCompletion(path.get("body"), context),
-    );
+    // @ts-expect-error(flow->ts): todo
+    return getStatementListCompletion(path.get("body"), context);
   } else if (path.isFunction()) {
     return _getCompletionRecords(path.get("body"), context);
   } else if (path.isTryStatement()) {
     records = addCompletionRecords(path.get("block"), records, context);
     records = addCompletionRecords(path.get("handler"), records, context);
   } else if (path.isCatchClause()) {
-    records = addCompletionRecords(path.get("body"), records, context);
+    return addCompletionRecords(path.get("body"), records, context);
   } else if (path.isSwitchStatement()) {
-    records = completionRecordForSwitch(path.get("cases"), records, context);
+    return completionRecordForSwitch(path.get("cases"), records, context);
   } else if (path.isSwitchCase()) {
-    records = records.concat(
-      getStatementListCompletion(path.get("consequent"), {
-        canHaveBreak: true,
-        shouldPopulateBreak: false,
-        inCaseClause: true,
-      }),
-    );
+    return getStatementListCompletion(path.get("consequent"), {
+      canHaveBreak: true,
+      shouldPopulateBreak: false,
+      inCaseClause: true,
+    });
   } else if (path.isBreakStatement()) {
     records.push(BreakCompletion(path));
   } else {
@@ -495,7 +497,7 @@ export function getBindingIdentifierPaths(
   [x: string]: NodePath;
 } {
   const path = this;
-  let search = [].concat(path);
+  const search = [path];
   const ids = Object.create(null);
 
   while (search.length) {
@@ -517,7 +519,7 @@ export function getBindingIdentifierPaths(
 
     if (id.isExportDeclaration()) {
       const declaration = id.get("declaration");
-      if (declaration.isDeclaration()) {
+      if (t.isDeclaration(declaration)) {
         search.push(declaration);
       }
       continue;
@@ -537,8 +539,10 @@ export function getBindingIdentifierPaths(
       for (let i = 0; i < keys.length; i++) {
         const key = keys[i];
         const child = id.get(key);
-        if (Array.isArray(child) || child.node) {
-          search = search.concat(child);
+        if (Array.isArray(child)) {
+          search.push(...child);
+        } else if (child.node) {
+          search.push(child);
         }
       }
     }