I have an arrow function that looks like this (simplified):
const f = arg => { arg.toUpperCase(); };
But when I call it, I get undefined:
console.log(f("testing")); // undefined
Why?
Example:
const f = arg => { arg.toUpperCase(); };
console.log(f("testing"));(Note: This is meant to be a clean, canonical dupetarget for the specific issue with arrow functions above.)
When you use the function body version of an arrow function (with {}), there is no implied return. You have to specify it. When you use the concise body (no {}), the result of the body expression is implicitly returned by the function.
So you would write that either with an explicit return:
const f = arg => { return arg.toUpperCase(); };
// Explicit return ^^^^^^
or with a concise body:
const f = arg => arg.toUpperCase();
Examples:
const f1 = arg => { return arg.toUpperCase(); };
console.log(f1("testing"));
const f2 = arg => arg.toUpperCase();
console.log(f2("testing"));Slightly tangential, but speaking of {}: If you want the concise arrow's body expression to be an object initializer, put it in ():
const f = arg => ({prop: arg.toUpperCase()});