Employing distinct techniques for a union-typed variable in TypeScript

I'm currently in the process of converting a JavaScript library to TypeScript. One issue I've encountered is with a variable that can be either a boolean or an array.

This variable cannot be separated into two different variables because it's provided by external JSON data, and the data structure must remain consistent for user expectations.

The challenge I'm facing is that when the variable is an array, the methods specific to arrays are not available on a boolean type.

As a result, TypeScript throws an error stating: 

Error TS2339: Property 'push' does not exist on type 'boolean | string[]'.

var children: boolean | Array<string>;

children = [];
children.push('test');

Even this simple code snippet demonstrates the issue I'm encountering. How can I find a solution to this problem?

javascripttypescript

Answer №1

I'm facing an issue with my code that is similar to the one demonstrated here. Any suggestions on how to fix it?

TypeScript has a good understanding of JavaScript :)

The code snippet below works perfectly in the latest TypeScript version: 

let values: boolean | Array<string>;

values = [];
values.push('example');

This is due to the advanced code flow analysis done by TypeScript ;)

Additional Information

TypeScript version :

Older Version

If you prefer, you can use a type guard in the current stable release of TypeScript (although I recommend updating to the latest version). More information on type guards can be found here:

Answer №2

To handle different types effectively, you can implement type guards:

if (children instanceof Array) {
    children.push('test');
}

Alternatively, you can use:

if (typeof children === "boolean") {
    // children is a boolean
}

In scenarios where handling various types requires more complex actions than a simple console.log, it might be beneficial to delegate tasks to separate functions based on the type:

var children: boolean | Array<string>;
if (children instanceof Array) {
    useChildrenArray(children);
} else {
    useChildrenBoolean(children);
}

function useChildrenBoolean(children: boolean) { ... }
function useChildrenArray(children: Array<string>) { ... }

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