String variable value as object key errors on IE11

Question

I have experienced a problem on a page under IE11, which crashes a page there. The problematic code essentially takes a String dynamically from the server, initializes a variable with that and later uses it as object key. To make the question digestible, I have written a small script:

var bar = "b";
var foo = {[bar]: "Orson oson a borsos borsós fosos koros boros korsón"};

This works under Chrome, FireFox and Microsoft Edge alike, but under IE11, I get the following error:

Expected identifier, string or number

Why is IE11 not allowing this and how should I solve this?

Maybe the code is ES6 or ES7 related. Try var bar = "b", foo = {}; foo[bar] = "Orson oson a borsos borsós fosos koros boros korsón"; — Mr_Green
– Mr_Green, Commented Oct 19, 2016 at 11:39
@Mr_Green, that works, but I wonder about the cause my code throws the error. — Lajos Arpad
– Lajos Arpad, Commented Oct 19, 2016 at 11:40
IE11 supports ES5 but not new versions like ES6 or ES7. In ES5 you can't place a variable in key of object. Hence, the error. — Mr_Green
– Mr_Green, Commented Oct 19, 2016 at 11:42
Maybe I don't understand what you're trying to do, but why not just doing key = String([bar]) and then use key? — Ronen Ness
– Ronen Ness, Commented Oct 19, 2016 at 11:48
@Ness, that will have no syntax error, but if you console.log(foo); you will see that it has a member called key which should not be there and there is no member called b, which should be there. — Lajos Arpad
– Lajos Arpad, Commented Oct 19, 2016 at 11:54

Jaromanda X · Accepted Answer · 2016-10-19 11:51:30Z

IE11 is old and doesn't support that syntax ... syntactical changes in ES2015+ are not always easy to implement in old tired browsers - though with this you could do

var bar = "b";
var foo = {};
foo[bar] = "Orson oson a borsos borsós fosos koros boros korsón";

using babel, however, the code translates to (ES2015-loose preset)

var _foo;

var bar = "b";
var foo = (_foo = {}, _foo[bar] = "Orson oson a borsos borsós fosos koros boros korsón", _foo);

or (ES2015 preset)

function _defineProperty(obj, key, value) { 
    if (key in obj) { 
        Object.defineProperty(obj, key, { 
            value: value, 
            enumerable: true, 
            configurable: true, 
            writable: true 
        }); 
    } else { 
        obj[key] = value; 
    } 
    return obj; 
}

var bar = "b";
var foo = _defineProperty({}, bar, "Orson oson a borsos borsós fosos koros boros korsón");

This syntax is referred to as Computed property names

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