typescript-compiler-api.ts

import * as ts from 'typescript'

function makeFactorialFunction() {
  const functionName = ts.factory.createIdentifier('factorial')
  const paramName = ts.factory.createIdentifier('n')
  const parameter = ts.factory.createParameterDeclaration(
    undefined,
    undefined,
    undefined,
    paramName,
    undefined,
    ts.factory.createKeywordTypeNode(ts.SyntaxKind.NumberKeyword)
  )

  const condition = ts.factory.createBinaryExpression(
    paramName,
    ts.SyntaxKind.LessThanEqualsToken,
    ts.factory.createNumericLiteral(1)
  )
  const ifBody = ts.factory.createBlock([
    ts.factory.createReturnStatement(ts.factory.createNumericLiteral(1)),
  ])

  const decrementedArg = ts.factory.createBinaryExpression(
    paramName,
    ts.SyntaxKind.MinusToken,
    ts.factory.createNumericLiteral(1)
  )
  const decrementedFunction = ts.factory.createCallExpression(
    functionName,
    undefined,
    [decrementedArg]
  )
  const recurse = ts.factory.createBinaryExpression(
    paramName,
    ts.SyntaxKind.AsteriskToken,
    decrementedFunction
  )
  const statements = [
    ts.factory.createIfStatement(condition, ifBody),
    ts.factory.createReturnStatement(recurse),
  ]
  const func = ts.factory.createFunctionDeclaration(
    undefined,
    [ts.factory.createToken(ts.SyntaxKind.ExportKeyword)],
    undefined,
    functionName,
    undefined,
    [parameter],
    ts.factory.createKeywordTypeNode(ts.SyntaxKind.NumberKeyword),
    ts.factory.createBlock(statements, true)
  )

  const funcall = ts.factory.createVariableStatement(
    undefined,
    ts.factory.createVariableDeclarationList(
      [
        ts.factory.createVariableDeclaration(
          ts.factory.createIdentifier('res'),
          undefined,
          undefined,
          ts.factory.createCallExpression(functionName, undefined, [
            ts.factory.createNumericLiteral(10),
          ])
        ),
      ],
      ts.NodeFlags.Const
    )
  )

  ts.NodeFlags.Const

  const print = ts.factory.createExpressionStatement(
    ts.factory.createCallExpression(
      ts.factory.createIdentifier('console.log'),
      undefined,
      [ts.factory.createIdentifier('res')]
    )
  )

  return ts.factory.createSourceFile(
    [func, funcall, print],
    ts.factory.createToken(ts.SyntaxKind.EndOfFileToken),
    ts.NodeFlags.BlockScoped
  )
}

const resultFile = ts.createSourceFile('factorial.ts', '', ts.ScriptTarget.Latest)
const printer = ts.createPrinter()
const result = printer.printNode(
  ts.EmitHint.Unspecified,
  makeFactorialFunction(),
  resultFile
)
console.log(result)