Question 51416 in Javascript Question posted in
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Cannot read properties of undefined while testing if undefined (but it is defined)? – Javascript

I have the following chunk of Typescript code in a function, with variableInQuestion = 0 being defined outside of the function:

console.log(this.variableInQuestion);
this.arrayOfObjects.each(function(thing) {
    if (this.variableInQuestion !== undefined && thing.property == this.variableInQuestion) {
        ...
    }
});

The problem is that the console.log line prints out exactly what I would expect (the default value of 0), but I get an error for the third line saying Cannot read properties of undefined (reading 'variableInQuestion') at [[[location of the first variableInQuestion in the third line]]]. I don’t understand why it’s giving me a "cannot read properties of undefined" error while I am testing to see if it’s undefined.

EDIT: the .each thing is specific to the array of objects in question; that’s not the issue. If I run the following…

console.log(this.variableInQuestion);
this.arrayOfObjects.each(function(thing) {
    if (0 !== undefined && thing.property == 0) {
        ...
    }
});

It works exactly as I’d expect.

Tags:

2

Answers


  1. Chosen as BEST ANSWER
    0

    The way to address this problem is as follows:

    variableInQuestion = 0;

myFunction() {
    let placeholder = this.variableInQuestion;
    this.arrayOfObjects.each(function(thing) {
        if (placeholder !== undefined && thing.property == placeholder) {
            ...
        }
    });
}

    Special thanks to Pointy for identifying the problem!


  2. 0

    If you console.log your data, you’ll see

    class X {
    variableInQuestion = 123;
    arrayOfObjects = [1, 2, 3];

    example() {
        console.log(this.variableInQuestion);
        this.arrayOfObjects.forEach(function (thing) {
            console.log({
                this: this,
                thing: thing,
            })
        })
    }
}

new X().example()
// 123
// { "this": undefined, thing: 1 }
// { "this": undefined, thing: 2 }
// { "this": undefined, thing: 3 }

    because forEach doesn’t keep this value.

    Either use lambda

    this.arrayOfObjects.forEach((thing) => { console.log(this) })

    , keep this value

    let that = this;
this.arrayOfObjects.forEach((thing) => { console.log(that) })

    , or provide thisArg into forEach (dunno about .each but Array.prototype.forEach supports that)

    this.arrayOfObjects.forEach(function(thing){ console.log(this) }, this)
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