esLint throws an error advising that a for-in loop should be enclosed within an if statement in order to exclude unnecessary properties from the prototype

Question

esLint throws an error advising that a for-in loop should be enclosed within an if statement in order to exclude unnecessary properties from the prototype

While working on my Angular project, I encountered an error with esLint related to the code snippet below:

private calculateFieldValue(value: any): any {
                let isEmptyObject = false;
                if (value && Array.isArray(value) && value.length === 1 && value[0]) {
                    for (const subAttr in value[0]) {
                        if (value[0].hasOwnProperty(subAttr)) {
                            if (value[0][subAttr]) {
                                break;
                            }
                        }
                        isEmptyObject = true;
                    }
                }
                return isEmptyObject ? null : value;
            }

The specific error that occurred is:

Error: The body of a for-in loop should be wrapped in an if statement to filter unwanted properties from the prototype chain. (guard-for-in)

angular typescript eslint lint jslint

Answer 1

Answer №1

Despite having a check for hasOwnProperty, it does not encompass the entirety of the loop body; isEmptyObject = true; will still run even if value[0] lacks that property. The linter requires you to execute statements in the body only if hasOwnProperty is successful.

for (const subAttr in value[0]) {
    if (value[0].hasOwnProperty(subAttr)) {
        if (value[0][subAttr]) {
            break;
        }
        isEmptyObject = true;
    }
}

I strongly advise against using any as it undermines TypeScript's purpose by essentially disabling various types of type-checking. Since you are returning either the array or null, utilize generics to specify the type of the array instead of resorting to any.

An alternative approach would be to employ Object.values to determine if any values are truthy - your focus is solely on the values, not the keys.

function getSubarrayObjectWithTruthyProperty<T extends unknown>(value: Array<T> | unknown) {
    if (value && Array.isArray(value) && value.length === 1) {
        const item: unknown = value[0];
        if (item && typeof item === 'object' && Object.values(item).some(Boolean)) {
            return value as T;
        }
    }
    return null;
}

Answer 2

Despite having a check for hasOwnProperty, it does not encompass the entirety of the loop body; isEmptyObject = true; will still run even if value[0] lacks that property. The linter requires you to execute statements in the body only if hasOwnProperty is successful.

for (const subAttr in value[0]) {
    if (value[0].hasOwnProperty(subAttr)) {
        if (value[0][subAttr]) {
            break;
        }
        isEmptyObject = true;
    }
}

I strongly advise against using any as it undermines TypeScript's purpose by essentially disabling various types of type-checking. Since you are returning either the array or null, utilize generics to specify the type of the array instead of resorting to any.

An alternative approach would be to employ Object.values to determine if any values are truthy - your focus is solely on the values, not the keys.

function getSubarrayObjectWithTruthyProperty<T extends unknown>(value: Array<T> | unknown) {
    if (value && Array.isArray(value) && value.length === 1) {
        const item: unknown = value[0];
        if (item && typeof item === 'object' && Object.values(item).some(Boolean)) {
            return value as T;
        }
    }
    return null;
}

esLint throws an error advising that a for-in loop should be enclosed within an if statement in order to exclude unnecessary properties from the prototype

Answer №1

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