This function is supposed to return an object, however, the construction used below is unfamiliar to me. How does this function work?
function expect(value) {
return {
toBe: exp => console.log(success)
}
}
2 Answers
This is a standard JavaScript function:
function(parameter1, parameter2) {
return returnVal;
}
But the object that is being returned looks like this:
{
toBe: exp => console.log(success)
}
Which is an object containing an ES6 arrow function, and can be alternatively expressed like so (a direct translation to an ES5 function):
{
toBe: function(exp) {
return console.log(success);
}
}
Read here for more information on ES6 arrow function notation.
2 Comments
Mulan
a more accurate es5 representation would be
return console.log(success). why did you add an explicit return undefined when the es6 isn't doing that?
Jack Bashford
My bad, got mixed up @Mulan (although it is functionally identical, only syntactically different).
I think it worth emphasizing that returns an object with a key that contains a function as a value. You can run it in the same way to run a method belonging to a class. Obliviously it fails because there is no success defined.
function expect(value) {
return {
toBe: exp => console.log(success)
}
}
let res = expect('value')
console.log(res)
res.toBe('test')I would say that this is the intent of the code, imo this makes a lot of sense; the evaluation is done when toBe is invoked, this invokes console.log(which tests the condition, when is invoked and prints the result):
function expect(value) {
return {
toBe: exp => console.log(exp === value)
}
}
expect('value').toBe('value')
expect('notvalue').toBe('value')
{}is after thereturn)?valueis never used, but the gist is: callexpect()and the return is an object with a single property:toBe, which is another function that ignores what it is passed and simply logs "success" to the console.