The provided code demonstrates TypeScript focusing on the line result[key].push(submenuKey); where it acknowledges that result might be null, but a check for Array.isArray() is included.
Code snippet:
interface resultI {
[key: string]: string | null | any[];
}
const result: resultI = {
fookey: ['foo', 'bar']
};
const key = 'fookey';
const submenuKey = 'submenuKey';
if (Array.isArray(result[key])) {
result[key].push(submenuKey);
}
How does this work exactly?
The reason behind the need for Array.isArray() is to protect the outcome of the result[key] expression. The second instance of result[key] represents a distinct expression that lacks protection (TypeScript cannot determine if it's guarded). To address this issue, you can store the value of result[key] in a variable and then safeguard the variable, as shown below:
...
const arr = result[key];
if (Array.isArray(arr)) {
arr.push(submenuKey);
}
This situation arises because the key is a variable. In the case of a fixed key, the guard functions effectively, illustrated by the following example:
if (Array.isArray(result['fookey'])) {
result['fookey'].push(submenuKey); // no errors
}
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