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I have a variable foo to be initialized. My attempt below does not compile.

let foo: (x: number)=>{y:number,z: number}= (x)=>{x+1,x+2};

with the following error:

Left side of comma operator is unused and has no side effects.ts(2695) (parameter) x: number

Question

What is the correct way to initialize the variable foo?

Second Person Shooter
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    `{x+1,x+2}` isn’t even a proper object literal. It should be `({ y: x + 1, z: x + 2 })`. See [ECMAScript 6 arrow function that returns an object](/q/28770415/4642212). – Sebastian Simon Oct 29 '21 at 11:21

1 Answers1

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Because of how arrow function notation works, the curly braces around the output are interpreted as containing the function body instead of a value being returned.

There are two ways around this. One is to use the longer arrow function notation, with a return statement. The other is to wrap your object in brackets.

It also looks like you've forgot to add the y and z labels in your output.

Try one of these:

let foo: (x: number) => {y: number, z: number} = (x) => ({y: x+1, z: x+2});

let foo: (x: number) => {y: number, z: number} = (x) => {
    return {y: x+1, z: x+2};
};

It's also worth noting that when initialising a variable in this way, TypeScript is able to infer its type. So you can actually shorten your code to just one of these:

let foo = (x: number) => ({y: x+1, z: x+2});

let foo = (x: number) => {
    return {y: x+1, z: x+2};
};
Mark Hanna
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