create.ts

// Creating and Printing a TypeScript AST
// https://github.com/Microsoft/TypeScript/wiki/Using-the-Compiler-API#creating-and-printing-a-typescript-ast

// outputs:
// export function factorial(n): number {
//     if (n <= 1) {
//         return 1;
//     }
//     return n * factorial(n - 1);
// }

import ts = require("typescript");

function makeFactorialFunction() {
    const functionName = ts.createIdentifier("factorial");
    const paramName = ts.createIdentifier("n");
    const parameter = ts.createParameter(
        /*decorators*/ undefined,
        /*modifiers*/ undefined,
        /*dotDotDotToken*/ undefined,
        paramName);

    const condition = ts.createBinary(
        paramName,
        ts.SyntaxKind.LessThanEqualsToken,
        ts.createLiteral(1));

    const ifBody = ts.createBlock(
        [ts.createReturn(ts.createLiteral(1))],
        /*multiline*/ true)
    const decrementedArg = ts.createBinary(paramName, ts.SyntaxKind.MinusToken, ts.createLiteral(1))
    const recurse = ts.createBinary(
        paramName,
        ts.SyntaxKind.AsteriskToken,
        ts.createCall(functionName, /*typeArgs*/undefined, [decrementedArg]));
    const statements = [
        ts.createIf(condition, ifBody),
        ts.createReturn(
            recurse
        ),
    ];

    return ts.createFunctionDeclaration(
        /*decorators*/ undefined,
        /*modifiers*/[ts.createToken(ts.SyntaxKind.ExportKeyword)],
        /*asteriskToken*/ undefined,
        functionName,
        /*typeParameters*/ undefined,
        [parameter],
        /*returnType*/ ts.createKeywordTypeNode(ts.SyntaxKind.NumberKeyword),
        ts.createBlock(statements, /*multiline*/ true),
    )
}

const resultFile = ts.createSourceFile("someFileName.ts", "", ts.ScriptTarget.Latest, /*setParentNodes*/ false, ts.ScriptKind.TS);
const printer = ts.createPrinter({
    newLine: ts.NewLineKind.LineFeed,
});
const result = printer.printNode(ts.EmitHint.Unspecified, makeFactorialFunction(), resultFile);

console.log(result);