Object initializer syntax in Array.prototype.map()

Question

let a = [{num:'1'},{num:'2'}];
let b = ['1','2'];
let c = b.map(i=>{num:i});

console.log(a);
console.log(c);

I expected variable c to be identical to a, which doesn't happen. Why is the Object initializer syntax not allowed as I'm using it?

Your object initializer is being treated like a code block. Use parentheses for it to be evaluated as an expression. ({num:i}) — castletheperson
– castletheperson, Commented Dec 16, 2016 at 22:49
what 4castle said. Furthermore, it's not a problem with map but in general with arrow functions. — VLAZ
– VLAZ, Commented Dec 16, 2016 at 22:49
As a perhaps irrelevant side note. The example initializes a new array with incrementing numbers. The same can be done with let c = Array.from({length:2},(o,i)=>({num:i+1})); (Same goes for parentheses around the arrow function though.) For 2 entries it doesn't matter, but for larger quantities it makes life easier. Likely it was just an example, but thought I'd mention it just in case — Me.Name
– Me.Name, Commented Dec 16, 2016 at 23:01

lonesomeday · Accepted Answer · 2016-12-16 22:47:45Z

4

The problem is the classic Javascript one: the contents of your arrow function are being treated as a block, not as a function literal, with num as a label, not as an object key. You need to wrap the literal in brackets () to make it work:

let a = [{num:'1'},{num:'2'}];
let b = ['1','2'];
let c = b.map(i=>({num:i}));

console.log(a);
console.log(c);

answered Dec 16, 2016 at 22:47

lonesomeday

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