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A JavaScript function runs when it is called. To call a function, write the name followed by parentheses like name().
NOT executed when the function is defined. The code inside a function will execute when "something" invokes the function:
When an event occurs (a user clicks a button) Automatically (self invoked) It is common to use the term invoke, because a function can be invoked without being called. It is also common to use say: call a function call upon a function start a function execute a function
"Hello World". But it will not run before you call it.
Example function sayHello() {
return "Hello World";
}The code below calls the function. But it does not use the result.
Example function sayHello() {
return "Hello World";
}
sayHello();To use the returned value, you can store it in a variable or display it.
When a function returns a value, you can store the value in a variable.
Example function sayHello() {
return "Hello World";
}
let greeting = sayHello();You can display the returned value in many ways, for example using console.log() or HTML.Examples function sayHello() {
return "Hello World";
}
console.log(sayHello());
<p id="demo"></p><script>
function sayHello() {
return "Hello World";
}
document.getElementById("demo").innerHTML = sayHello();</script>
You can call the same function whenever you need it.
Example function sayHello() {
return "Hello World";
}
let a = sayHello();
let b = sayHello();
let c = sayHello();
Functions are Invoked with ()
The () operator invokes a function.Example toCelsius() invokes the toCelsius function: // Convert Fahrenheit to Celsius:
function toCelsius(fahrenheit) {
return (5/9) * (fahrenheit-32);
}
// Call the toCelcius() function let value = toCelsius(77);
Accessing a function without (), returns the function itself and not the result:Example function toCelsius(fahrenheit) {
return (5/9) * (fahrenheit-32);
}
let value = toCelsius;In the example above toCelsius refers to the function itself. toCelsius(77) refers to the function result.
sayHello refers to the function itself. It returns the function. sayHello() refers to the function result. It returns the result
Example function sayHello() {
return "Hello World";
}
let text = sayHello;In the example above, text returns the function itself like:
function sayHello() { return "Hello World"; }You can call functions from other functions, from events, or from any code block.
Examples function sayHello() {
return "Hello World";
}
function showHello() {
document.getElementById("demo").innerHTML = sayHello();
}
<p id="demo"></p>
<button onclick="showHello()">Click Me</button><script>
function sayHello() {
return "Hello World";
}
function showHello() {
document.getElementById("demo").innerHTML = sayHello();
}</script>
Forgetting Parentheses () sayHello does not run the function. You must use sayHello().
Some functions does not return a value.If a function returns a value, but you must display it to see it. What is the difference between sayHello and sayHello() ?
Parameters allow you to send values to a function Parameters are listed in parentheses in the function definition