Using spread syntax and new Set() with typescript

问题

I am using following code to get unique numbers:

let uniques = [ ...new Set([1, 2, 3, 1, 1]) ]; // [1, 2, 3]

However, typescript report following error: Type 'Set' is not an array type. I am not typescript ninja, could someone tell me what is wrong here?

回答1:

This is a missing feature. TypeScript only supports iterables on Arrays at the moment.

回答2:

Update: With Typescript 2.3, you can now add "downlevelIteration": true to your tsconfig, and this will work while targeting ES5.

The downside of downlevelIteration is that TS will have to inject quite a bit of boilerplate when transpiling. The single line from the question transpiles with 21 lines of added boilerplate: (as of Typescript 2.6.1)

var __read = (this && this.__read) || function (o, n) {
    var m = typeof Symbol === "function" && o[Symbol.iterator];
    if (!m) return o;
    var i = m.call(o), r, ar = [], e;
    try {
        while ((n === void 0 || n-- > 0) && !(r = i.next()).done) ar.push(r.value);
    }
    catch (error) { e = { error: error }; }
    finally {
        try {
            if (r && !r.done && (m = i["return"])) m.call(i);
        }
        finally { if (e) throw e.error; }
    }
    return ar;
};
var __spread = (this && this.__spread) || function () {
    for (var ar = [], i = 0; i < arguments.length; i++) ar = ar.concat(__read(arguments[i]));
    return ar;
};
var uniques = __spread(new Set([1, 2, 3, 1, 1]));
console.log(uniques);

This boilerplate will be injected once per file that uses downlevel iteration, and this boilerplate can be reduced using the "importHelpers" option via the tsconfig. (See this blogpost on downlevel iteration and importHelpers)

Alternatively, if ES5 support doesn't matter for you, you can always just target "es6" in the first place, in which case the original code works without needing the "downlevelIteration" flag.

Original answer:

This seems to be a typescript ES6 transpilation quirk . The ... operator should work on anything that has an iterator property, (Accessed by obj[Symbol.iterator]) and Sets have that property.

To work around this, you can use Array.from to convert the set to an array first: ...Array.from(new Set([1, 2, 3, 1, 1])).

回答3:

You can also use Array.from method to convert the Set to Array

let uniques = Array.from(new Set([1, 2, 3, 1, 1])) ;
console.log(uniques);

回答4:

You need to set "target": "es6", in your tsconfig.

回答5:

To make it work, you either need "target": "ES6" (or higher) or "downlevelIteration": true in the compilerOptions of your tsconfig.json . This resolved my issue and working good or me.Hope it will help you also.

来源：https://stackoverflow.com/questions/33464504/using-spread-syntax-and-new-set-with-typescript

标签

javascript

typescript

ecmascript-6

spread-syntax