Extract 'getBlockStringIndentation' function

src/language/__tests__/blockString-test.js

@@ -9,7 +9,11 @@
 
 import { expect } from 'chai';
 import { describe, it } from 'mocha';
-import { dedentBlockStringValue, printBlockString } from '../blockString';
+import {
+  dedentBlockStringValue,
+  getBlockStringIndentation,
+  printBlockString,
+} from '../blockString';
 
 function joinLines(...args) {
   return args.join('\n');
@@ -99,6 +103,37 @@ describe('dedentBlockStringValue', () => {
   });
 });
 
+describe('getBlockStringIndentation', () => {
+  it('returns zero for an empty array', () => {
+    expect(getBlockStringIndentation([])).to.equal(0);
+  });
+
+  it('do not take first line into account', () => {
+    expect(getBlockStringIndentation(['  a'])).to.equal(0);
+    expect(getBlockStringIndentation([' a', '  b'])).to.equal(2);
+  });
+
+  it('returns minimal indentation length', () => {
+    expect(getBlockStringIndentation(['', ' a', '  b'])).to.equal(1);
+    expect(getBlockStringIndentation(['', '  a', ' b'])).to.equal(1);
+    expect(getBlockStringIndentation(['', '  a', ' b', 'c'])).to.equal(0);
+  });
+
+  it('count both tab and space as single character', () => {
+    expect(getBlockStringIndentation(['', '\ta', '          b'])).to.equal(1);
+    expect(getBlockStringIndentation(['', '\t a', '          b'])).to.equal(2);
+    expect(getBlockStringIndentation(['', ' \t a', '          b'])).to.equal(3);
+  });
+
+  it('do not take empty lines into account', () => {
+    expect(getBlockStringIndentation(['a', '\t'])).to.equal(0);
+    expect(getBlockStringIndentation(['a', ' '])).to.equal(0);
+    expect(getBlockStringIndentation(['a', ' ', '  b'])).to.equal(2);
+    expect(getBlockStringIndentation(['a', ' ', '  b'])).to.equal(2);
+    expect(getBlockStringIndentation(['a', '', ' b'])).to.equal(1);
+  });
+});
+
 describe('printBlockString', () => {
   it('by default print block strings as single line', () => {
     const str = 'one liner';

src/language/blockString.js

@@ -18,22 +18,9 @@ export function dedentBlockStringValue(rawString: string): string {
   const lines = rawString.split(/\r\n|[\n\r]/g);
 
   // Remove common indentation from all lines but first.
-  let commonIndent = null;
-  for (let i = 1; i < lines.length; i++) {
-    const line = lines[i];
-    const indent = leadingWhitespace(line);
-    if (
-      indent < line.length &&
-      (commonIndent === null || indent < commonIndent)
-    ) {
-      commonIndent = indent;
-      if (commonIndent === 0) {
-        break;
-      }
-    }
-  }
+  const commonIndent = getBlockStringIndentation(lines);
 
-  if (commonIndent) {
+  if (commonIndent !== 0) {
     for (let i = 1; i < lines.length; i++) {
       lines[i] = lines[i].slice(commonIndent);
     }
@@ -51,6 +38,28 @@ export function dedentBlockStringValue(rawString: string): string {
   return lines.join('\n');
 }
 
+// @internal
+export function getBlockStringIndentation(lines: Array<string>): number {
+  let commonIndent = null;
+
+  for (let i = 1; i < lines.length; i++) {
+    const line = lines[i];
+    const indent = leadingWhitespace(line);
+    if (indent === line.length) {
+      continue; // skip empty lines
+    }
+
+    if (commonIndent === null || indent < commonIndent) {
+      commonIndent = indent;
+      if (commonIndent === 0) {
+        break;
+      }
+    }
+  }
+
+  return commonIndent === null ? 0 : commonIndent;
+}
+
 function leadingWhitespace(str) {
   let i = 0;
   while (i < str.length && (str[i] === ' ' || str[i] === '\t')) {