Javascript for dummies 2019 pdf download

In most cases, you can omit semicolons altogether from your programs without even thinking about it. This fact is very controversial. Some developers will always use semicolons, some others will never use semicolons, and you'll always find code that uses semicolons and code that does not. A hello string is a value. A number like 12 is a value. The type is the kind of value, its category. We have many different types in JavaScript, and we'll talk about them in detail later on.

Each type has its own characteristics. When we need to have a reference to a value, we assign it to a variable. The variable can have a name, and the value is what's stored in a variable, so we can later access that value through the variable name. A variable is a value assigned to an identifier, so you can reference and use it later in the program. This is because JavaScript is loosely typed , a concept you'll frequently hear about.

This means the reference cannot be changed. You cannot reassign a new value to it. In particular, it does not mean the value cannot change - it means it cannot be reassigned. If the variable points to an object or an array we'll see more about objects and arrays later the content of the object or the array can freely change.

My advice is to always use const and only use let when you know you'll need to reassign a value to that variable. Because the less power our code has, the better. If we know a value cannot be reassigned, it's one less source for bugs. Now that we saw how to work with const and let , I want to mention var. Until , var was the only way we could declare a variable in JavaScript. Today, a modern codebase will most likely just use const and let. There are some fundamental differences which I detail in this post but if you're just starting out, you might not care about them.

Just use const and let. Once you assign a value with some type to a variable, you can later reassign the variable to host a value of any other type without any issues. In JavaScript we have 2 main kinds of types: primitive types and object types. Any value that's not of a primitive type a string, a number, a boolean, null or undefined is an object.

Object types have properties and also have methods that can act on those properties. An expression is a single unit of JavaScript code that the JavaScript engine can evaluate, and return a value.

Arithmetic expressions are expressions that take a variable and an operator more on operators soon , and result in a number:. Operators allow you to get two simple expressions and combine them to form a more complex expression. We can classify operators based on the operands they work with.

Some operators work with 1 operand. Most work with 2 operands. Just one operator works with 3 operands. In this first introduction to operators, we'll introduce the operators you are most likely familiar with: operators with 2 operands. Let's now introduce another set of binary operators that you're already familiar with from basic math.

If you divide by zero, JavaScript does not raise any error but returns the Infinity value or -Infinity if the value is negative. A remainder by zero is always NaN , a special value that means "Not a Number":. Every complex statement with multiple operators in the same line will introduce precedence problems. Some operations have more precedence than the others. The precedence rules are listed in this table:. After assignment and math operators, the third set of operators I want to introduce is conditional operators.

Comparison operators always return a boolean, a value that's true or false. In addition to those, we have 4 equality operators. They accept two values, and return a boolean:. An if statement is used to make the program take a route, or another, depending on the result of an expression evaluation. The conditional checks the expression you pass to it for a true or false value. If you pass a number, that always evaluates to true unless it's 0. If you pass a string, it always evaluates to true unless it's an empty string.

Those are general rules of casting types to a boolean. Did you notice the curly braces? That is called a block , and it is used to group a list of different statements. A block can be put wherever you can have a single statement. And if you have a single statement to execute after the conditionals, you can omit the block, and just write the statement:. The first is using the array literal syntax.

The second uses the Array built-in function. Since we can add an array into an array, we can create multi-dimensional arrays, which have very useful applications e. You can initialize a new array with a set of values using this syntax, which first initializes an array of 12 elements, and fills each element with the number 0 :.

Note that you can set the length of the array. If you assign a bigger number than the arrays current capacity, nothing happens. If you assign a smaller number, the array is cut at that position:. Returns the first item that returns true, and returns undefined if the element is not found. I personally prefer single quotes all the time, and use double quotes only in HTML to define attributes.

Another way to define strings is to use template literals, defined inside backticks. They are especially useful to make multiline strings much simpler. With single or double quotes you can't define a multiline string easily - you'd need to use escaping characters. Once a template literal is opened with the backtick, you just press enter to create a new line, with no special characters, and it's rendered as-is:. Main course contains 2 parts which cover JavaScript as a programming language and working with a browser.

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In the JavaScript cheat sheet above, we have compiled many of the most basic and important operators, functions, principles, and methods. It provides a good overview of the language and a reference for both developers and learners. We hope you have found it useful. Do you have additions to the JavaScript cheat sheet? Please let us know in the comments section below! Twitter Facebook. Last updated: Sep 3, 26 Comments. Below you can find the Javascript cheat sheet in. It can be reassigned but only accessed within a function.

Variables defined with var move to the top when the code is executed. Data Types Variables can contain different types of values and data types. Objects Objects are certain kinds of variables. Array Methods Once you have created arrays, there are a few things you can do with them: concat — Join several arrays into one indexOf — Returns the first position at which a given element appears in an array join — Combine elements of an array into a single string and return the string lastIndexOf — Gives the last position at which a given element appears in an array pop — Removes the last element of an array push — Add a new element at the end reverse — Sort elements in a descending order shift — Remove the first element of an array slice — Pulls a copy of a portion of an array into a new array sort — Sorts elements alphabetically splice — Adds elements in a specified way and position toString — Converts elements to strings unshift —Adds a new element to the beginning valueOf — Returns the primitive value of the specified object Operators If you have variables, you can use them to perform different kinds of operations.

Outputting Data A common application for functions is the output of data. Strings Strings are what JavaScript calls to text that does not perform a function but can appear on the screen.

Escape Characters In JavaScript, strings are marked with single or double-quotes. Setting Dates Date — Creates a new date object with the current date and time Date , 5, 21, 3, 23, 10, 0 — Create a custom date object.