javascriptScope errorHard

Counter method crashes inside forEach

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What it should do

countAll(items) counts the items using a Counter object. Example: countAll([1, 2, 3]) → 3.

What happens instead

countAll([1, 2, 3]) throws TypeError: Cannot read properties of undefined (reading 'n').

class Counter {
    constructor() {
        this.n = 0;
    }
    inc() {
        this.n = this.n + 1;
    }
}

function countAll(items) {
    const counter = new Counter();
    items.forEach(counter.inc);
    return counter.n;
}

Test a custom input to verify an edge case (e.g. empty lists, negative numbers, duplicates).

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This bug belongs to the scope and closure bugs pattern

Every callback reports the last value, a counter never increments, UnboundLocalError on a variable you can see.

Learn the scope and closure bugs pattern Step through your own code

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class Counter { constructor() { this.n = 0; } inc() { this.n = this.n + 1; } } function countAll(items) { const counter = new Counter(); items.forEach(counter.inc); return counter.n; }

Counter method crashes inside forEach

What it should do

What happens instead

More scope and closure bugs to practise

Seen this error in your own code?

Counter method crashes inside forEach

What it should do

What happens instead

More scope and closure bugs to practise

Seen this error in your own code?