JavaScript is a programming language that adds interactivity to your website. This happens in games, in the behavior of responses when buttons are pressed or with data entry on forms; with dynamic styling; with animation, etc. This article helps you get started with JavaScript and furthers your understanding of what is possible.
The control flow is the order in which the computer executes statements in a script.
Code is run in order from the first line in the file to the last line, unless the computer runs across the (extremely frequent) structures that change the control flow, such as conditionals and loops.
For example, imagine a script used to validate user data from a webpage form. The script submits validated data, but if the user, say, leaves a required field empty, the script prompts them to fill it in. To do this, the script uses a conditional structure or if…else, so that different code executes depending on whether the form is complete or not:
if (field==empty) { promptUser(); } else { submitForm(); } A typical script in JavaScript or PHP (and the like) includes many control structures, including conditionals, loops and functions. Parts of a script may also be set to execute when events occur.
For example, the above excerpt might be inside a function that runs when the user clicks the Submit button for the form. The function could also include a loop, which iterates through all of the fields in the form, checking each one in turn. Looking back at the code in the if and else sections, the lines promptUser and submitForm c
Functions are a way of packaging functionality that you wish to reuse. It’s possible to define a body of code as a function that executes when you call the function name in your code. This is a good alternative to repeatedly writing the same code. You have already seen some uses of functions previously. For example:
let myVariable = document.querySelector(‘h1’); alert(‘hello!’); These functions, document.querySelector and alert, are built into the browser.
If you see something which looks like a variable name, but it’s followed by parentheses— () —it is likely a function. Functions often take arguments: bits of data they need to do their job. Arguments go inside the parentheses, separated by commas if there is more than one argument.
For example, the alert() function makes a pop-up box appear inside the browser window, but we need to give it a string as an argument to tell the function what message to display.
You can also define your own functions. In the next example, we create a simple function which takes two numbers as arguments and multiplies them:
function multiply(num1,num2) { let result = num1 * num2; return result; }
An assignment operator assigns a value to its left operand based on the value of its right operand. The simple assignment operator is equal (=), which assigns the value of its right operand to its left operand. That is, x = y assigns the value of y to x.
There are also compound assignment operatorsfor example:
Logical operators are typically used with Boolean (logical) values; when they are, they return a Boolean value. However, the && and || operators actually return the value of one of the specified operands, so if these operators are used with non-Boolean values, they may return a non-Boolean value. Operator Usage Description
| Logical OR ( | ) expr1 | expr2 Returns expr1 if it can be converted to true; otherwise, returns expr2. Thus, when used with Boolean values, | returns true if either operand is true; if both are false, returns false. |