javascript function and foreach scopes

Question

I'm having some issue with the js strange scope behaviour. I have this Graph function (a simple wrapper of https://github.com/cpettitt/graphlib). If I add nodes one by one like I do with the node labeled "1" all works, if I use a forEach I have a scope problem.

function Graph() {
    this.g = new graphlib.Graph();
    this.setNode = function (id, value) {
        this.g.setNode(id, value || {}); 
    }
}

var gg = new Graph();
    gg.setNode("1"); //works

    ["2a","2b"].forEach(gg.setNode); //doesn't work

The error is Cannot read property 'g' of undefined the g instance var of Graph isn't visible in that forEach.

No, the error means that `this` is `undefined` (you are trying to access the property `g` on `this` (`this.g`), "Cannot read property 'g' of undefined" therefore means that `this` is `undefined`). While `this` and "scope" are related, the problem is with `this`, not the scope. — Felix Kling, Jan 14 '17 at 23:16

score 0 · Accepted Answer · answered Jan 14 '17 at 23:16

0

Use the second argument to forEach to lock the function call to a scope:

["2a","2b"].forEach(gg.setNode, gg);

answered Jan 14 '17 at 23:16

Ates Goral

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`this` and scope are related but not the same. – Felix Kling Jan 14 '17 at 23:19
it works. Ates Goral and @Felix Kling could you provide some references/articles about the unusual js this/scope stuff? – alfredopacino Jan 14 '17 at 23:23
@alfredopacino: See the duplicate question. – Felix Kling Jan 14 '17 at 23:23

javascript function and foreach scopes

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