CODEWITHSUNDEEP
reactjstypescriptecmascript-5
unknown_boundaries
unknown_boundaries

Reputation: 1600

How should I fix "Expected to return a value at the end of arrow function."?

I'm using typescript and eslint. Eslint complaing about return after => arrow, when I added it this didn't work either - return new Promise((resolve, reject) => return {}. What is the correct syntax for -

function getSizeFromObjectUrl(dataURL: string): Promise<any> {
    return new Promise((resolve, reject) => {
        try {
            const img = new Image();
            img.onload = () => {
                const ratio = Math.min(300.0 / img.width, 300.0 / img.height);
                return resolve({
                    height: img.height * ratio,
                    width: img.width * ratio
                });
            };
            img.src = dataURL;
        } catch (exception) {
            return reject(exception);
        }
    });
}

using it like -

const size = await getSizeFromObjectUrl(imageUrl);

Upvotes: 0

Views: 1027

Answers (3)

AqueleHugo
AqueleHugo

Reputation: 308

The rule is about consistent return: https://eslint.org/docs/rules/consistent-return

A confusing aspect of JavaScript is that a function returns undefined if any of the following are true:

  • it does not execute a return statement before it exits
  • it executes return which does not specify a value explicitly
  • it executes return undefined
  • it executes return void followed by an expression (for example, a function call)
  • it executes return followed by any other expression which evaluates to undefined

If any code paths in a function return a value explicitly but some code path do not return a value explicitly, it might be a typing mistake, especially in a large function.

So what you need you can do to get rid of the message is:

  • explicitly return something in the try block (return undefined if you will)
  • stop returning in the catch block
  • disable the rule for that function

Upvotes: 2

sh.alawneh
sh.alawneh

Reputation: 649

the correct syntax is:

function getSizeFromObjectUrl(dataURL: string): Promise<any> {
    return new Promise((resolve, reject) => {
        try {
            const img = new Image();
            img.onload = () => {
                const ratio = Math.min(300.0 / img.width, 300.0 / img.height);
                resolve({
                    height: img.height * ratio,
                    width: img.width * ratio
                });
            };
            img.src = dataURL;
        } catch (exception) {
            reject(exception);
        }
    });
}

it's actually complaining about the return before resolve / reject not after the arrow. cuz the resolve and reject functions are voids

for the error Unexpected lexical declaration in case block., use the case like this :

case x: {
    // your code goes here
}

instead of:

case x: 
    // your code

Upvotes: 0

unknown_boundaries
unknown_boundaries

Reputation: 1600

I removed return before promise resolve/reject. This works -

function getSizeFromObjectUrl(dataURL: string): Promise<any> {
    return new Promise((resolve, reject) => {
        try {
            const img = new Image();
            img.onload = () => {
                const ratio = Math.min(300.0 / img.width, 300.0 / img.height);
                resolve({
                    height: img.height * ratio,
                    width: img.width * ratio
                });
            };
            img.src = dataURL;
        } catch (exception) {
            reject(exception);
        }
    });
}

Upvotes: 0

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