Compile TypeScript to ES6 Code

When we do tsc --init to initialize a TypeScript project, it generates a tsconfig.json file. We can see a JSON object with compilerOptions property. We can generate ES6 output by setting target to ES6 or ES2015.

{
  "compilerOptions": {
    //...
    "target": "ES6"
  }
}

If our TypeScript code is:

let a = 10;

const obj = {
  name: 10,
};

const response = {
  ...obj,
  age: 23,
};

When we target to ES6, the output will be:

"use strict";
let a = 10;
const obj = {
  name: 10,
};
const response = Object.assign(Object.assign({}, obj), { age: 23 });

As you can see, the let is retained because let is available in ES6. Whereas object spread operator was release in ES2018. Therefore object spread operator is converted to ES5 way.

If the target is set to ES2018, the output will be:

"use strict";
let a = 10;
const obj = {
  name: 10,
};
const response = {
  ...obj,
  age: 23,
};