javascript - Transpiling ES6 to ES5 online with Babel - Stack Overflow

admin管理员组

文章数量:1405393

As you can see here I want to transpile following code from ES6 to ES5 and use as presets "es2015":

let myString = "Whatever";
let myStringArray = Array.from (myString);
console.log (myStringArray);

As you know there is no "Array.from" method in ES5 but the transpiled code still contains Array.from. What did I do wrong?

As you can see here I want to transpile following code from ES6 to ES5 and use as presets "es2015":

let myString = "Whatever";
let myStringArray = Array.from (myString);
console.log (myStringArray);

As you know there is no "Array.from" method in ES5 but the transpiled code still contains Array.from. What did I do wrong?

• javascript
• ecmascript-6
• babeljs
• transpiler

Share Improve this question Follow asked Jan 29, 2017 at 22:30 user3025289user3025289 4

• 3 See babeljs.io/docs/usage/polyfill . You didn't do anything wrong, Babel doesn't transpile APIs. – Felix Kling Commented Jan 29, 2017 at 22:32
• thank you but I am trying it online at the babel play ground and I want to get it transpiled there,.. – user3025289 Commented Jan 29, 2017 at 22:34
• I updated my ment that Babel doesn't transpile built-in APIs. As far as I can tell there is nothing you can do about it in the online tool. – Felix Kling Commented Jan 29, 2017 at 22:35
• 1 It is impossible to handle every case of Array.from in a transpiler, that is why polyfills exist. All you need to do is load a polyfill like the link above says. – loganfsmyth Commented Jan 30, 2017 at 1:24

Add a ment  | 

1 Answer 1

Sorted by: Reset to default 4

Include this polyfill snippet above your code, so you can transpile it on the fly online. But for production you should use babel-polyfill

// Production steps of ECMA-262, Edition 6, 22.1.2.1
// Reference: https://people.mozilla/~jorendorff/es6-draft.html#sec-array.from
if (!Array.from) {
  Array.from = (function () {
    var toStr = Object.prototype.toString;
    var isCallable = function (fn) {
      return typeof fn === 'function' || toStr.call(fn) === '[object Function]';
    };
    var toInteger = function (value) {
      var number = Number(value);
      if (isNaN(number)) { return 0; }
      if (number === 0 || !isFinite(number)) { return number; }
      return (number > 0 ? 1 : -1) * Math.floor(Math.abs(number));
    };
    var maxSafeInteger = Math.pow(2, 53) - 1;
    var toLength = function (value) {
      var len = toInteger(value);
      return Math.min(Math.max(len, 0), maxSafeInteger);
    };

    // The length property of the from method is 1.
    return function from(arrayLike/*, mapFn, thisArg */) {
      // 1. Let C be the this value.
      var C = this;

      // 2. Let items be ToObject(arrayLike).
      var items = Object(arrayLike);

      // 3. ReturnIfAbrupt(items).
      if (arrayLike == null) {
        throw new TypeError("Array.from requires an array-like object - not null or undefined");
      }

      // 4. If mapfn is undefined, then let mapping be false.
      var mapFn = arguments.length > 1 ? arguments[1] : void undefined;
      var T;
      if (typeof mapFn !== 'undefined') {
        // 5. else      
        // 5. a If IsCallable(mapfn) is false, throw a TypeError exception.
        if (!isCallable(mapFn)) {
          throw new TypeError('Array.from: when provided, the second argument must be a function');
        }

        // 5. b. If thisArg was supplied, let T be thisArg; else let T be undefined.
        if (arguments.length > 2) {
          T = arguments[2];
        }
      }

      // 10. Let lenValue be Get(items, "length").
      // 11. Let len be ToLength(lenValue).
      var len = toLength(items.length);

      // 13. If IsConstructor(C) is true, then
      // 13. a. Let A be the result of calling the [[Construct]] internal method of C with an argument list containing the single item len.
      // 14. a. Else, Let A be ArrayCreate(len).
      var A = isCallable(C) ? Object(new C(len)) : new Array(len);

      // 16. Let k be 0.
      var k = 0;
      // 17. Repeat, while k < len… (also steps a - h)
      var kValue;
      while (k < len) {
        kValue = items[k];
        if (mapFn) {
          A[k] = typeof T === 'undefined' ? mapFn(kValue, k) : mapFn.call(T, kValue, k);
        } else {
          A[k] = kValue;
        }
        k += 1;
      }
      // 18. Let putStatus be Put(A, "length", len, true).
      A.length = len;
      // 20. Return A.
      return A;
    };
  }());
}

本文标签： javascriptTranspiling ES6 to ES5 online with BabelStack Overflow