startsWith is not a function getting called for no reason

Question

I have this code:

for (var i = 0; i < value.length; i++) {
    if (typeof value[i].keyword == 'undefined' || value[i].keyword == null || value[i].keyword.startsWith(keyword)) {
        out.push(value[i]);
    }
}

I am getting an error message saying:

> TypeError: r[e].startsWith is not a function
>    at js-cf2cc68….min.js.gz:85
>    at fn (eval at compile (js-cf2cc68….min.js.gz:8), <anonymous>:4:1003)
>    at js-cf2cc68….min.js.gz:7
>    at p.$digest (js-cf2cc68….min.js.gz:7)
>    at p.$apply (js-cf2cc68….min.js.gz:7)
>    at HTMLBodyElement.<anonymous> (js-cf2cc68….min.js.gz:9)

How is this possible? I think I've accounted for everything.

Answer 1

I fixed this by calling value.toString().startsWith(...)

Answer 2

value[i].keyword.startsWith("keyword") because the parameter of start with must be a string.

So that will work better this way

for (var i = 0; i < value.length; i++) {
    if (typeof value[i].keyword == String(undefined) || value[i].keyword.startsWith("keyword"))
        out.push(value[i]);
}

Answer 3

Found a useful article on this topic

The three approaches for converting to string are:

1. value.toString()
2. "" + value
3. String(value)

The point to note here is that approach # 1 doesn’t work if the value is null or undefined.

In my case, approach # 2 for some reason did not work either so the best option would be String(value)

var col = "rt_" + rows[i]["results"][r].ResultTypeID.substring(1); //did not work

var col = "rt_" + String(rows[i]["results"][r].ResultTypeID).substring(1);

Answer 4

Is there a way that I can check if it's a string rather than have it error out?

checking the type?

var out = values.filter(v => v.keyword == null || typeof v.keyword === "string" && v.keyword.startsWith( keyword ));

or simply enforcing the type

var out = values.filter(v => v.keyword == null || String(v.keyword).startsWith( keyword ));

or if you use desctructuring, you can use even this:

var out = values.filter({keyword: v}) => v == null || String(v).startsWith( keyword ));

I'd reccomend you to use the Array-methods instead of manually writing loops.

• Imo. it is a better description of your intent (good for scanning over code).

• If this is a hot path (it's called often) the JS-optimizer can take advantage of knowing, that youre'just filtering, and optimize the code. Maybe by skipping the lots of integrity-checks for the Array it performs during your loop.

• And if this is not a hot-path, the performance-impact of calling a function a few times will even be hard to measure, in JS.

Answer 5

I assume value[i].keyword is a string. String.prototype.startWith is not supported in older browsers. See browser support.

To use it in older browsers, you can use one of existing polyfills. See also answers from How to check if a string “StartsWith” another string?