Question

I am trying to understand storing a function as a variable. If I have the following function:

function calc() {

     return 2 * 2 

}

calc() //returns 4

var test = calc() 

test // returns 4

I know that the variable test has the value of 4, which makes sense.

However, in this example:

  function foo() {

     console.log("HEY")

  }

 foo() //logs "HEY" in the console

 var test = foo() 

 test // does NOT log "HEY" in the console.

Why does that not follow the same pattern?

Answer 1

var test = foo() does not store the function foo into the variable test, it stores the return value of foo in test, and since foo doesn't return anything, the value stored in test will be undefined.

Further explanation: Placing parentheses after a function name or a reference to a function will call that function. To just reference the function without calling it, simply don't use parentheses until you're ready to call it. For example:

function foo() {
  console.log("HEY")
}

foo() // "HEY"

var test = foo; // no () here

test() // use () here. "HEY"

Answer 2

function foo() {

     console.log("HEY")

  }

 foo() //logs "HEY" in the console

//correct way
// foo() is calling the function and asigning its return value
// foo is the function
 var test = foo 
 
 //then we call the stored function
 test()

Answer 3

it's about returning and doing something!

when you call below fn, it's returning a value

var calc = function () { return 2*2; }

if you call this, it's doing something!

var foo = function () { console.log("hey"); }

that means, when you call calc fn, you need to assign it a variable

var result = calc();

if you want to log something, you just call foo (not assign)

foo(); // log appears on the fly

var bar = foo(); // log appears again but since foo is not return something bar will be nothing

// assign fn to variable, so now you can call variable!
bar = foo; // no log, wait for the call
bar(); // log appears!

Answer 4

You need to do: return "hey"; instead of console.log(hey);

Answer 5

// This does the console.log as soon as it is called - it does not store the response and wait until a later time.
function foo() {
    console.log("hey"); 
}

// If you wanted to console.log something when you assign the variable, you can do this:
function foo() {
    return "hey";
}
var test = foo();

console.log('test: ', test);

// or this:
function foo() {
    return "hey";
}
var test = console.log(foo());

JS: Storing a function as a variable

5 个答案: