An expression is an unit of code that JavaScript interpreter can parse and compute to produce a value.
There are simple expressions like literal values and complex which are built from simpler ones usually using operators.
Primary expressions
Primary expressions are the simplest expressions. There are three groups of primary expressions: literal values, variable references, and some keywords.
Literal values
Literal values are constant values:
"text" // a string literal 125 // a number literal
Variable references
Any identifier that appears in the code JavaScript interpreter assumes it is a variable and try to read its value.
x // evaluates to the value of the variable x price // evaluates to the value of the variable price undefined // a global variable undefined evaluates to the value undefined
Basic keywords and reserved words
The this
The this evaluates to the current object. It is used in a method to refer to the current object, for example:
this.name this.displayName()
Other examples:
true // evaluates to the boolean true false // evaluates to the boolean false null //evaluate to the null value.
Function expression
Function expression defines a JavaScript function and the value of this expression is newly defined function. For example:
var sum = function (x, y) {
return x + y;
}
var z = sum (3, 4); //7
Object initializer expression
Object initializer creates object with literal notation and the value of this expression is newly created object. It uses curly brackets surrounding object properties separated by commas. For example:
var obj = {
prop1: "value1",
prop2: 2
};
Array initializer expression
Array initializer creates array with literal notation and the value of this expression is newly created array. It consists of square brackets surrounding elements separated by commas. For example:
var arr = [1, 2, 3];
Object creation expression
Object creation expression creates a new instance of object. It uses the keyword new followed by a constructor invocation.
An example:
var obj = new Object();
Property access expression
There are two ways to access a property of an object: either using the object followed by a period and an identifier or using the object (or the array) followed by square brackets with an identifier inside. It evaluates respectively to the value of an object property or an array element.
var obj = {x: 1, y: 2};
obj.x // 1
obj['y'] // 2
var arr = [2, 3];
arr[1] // 3
Invocation expression
Invocation expression is used to call a function or a method in JavaScript.
func(arg); obj.myMeth(x, y);
Parent object reference expression
Parent object reference is a new added in ECMAScript 2015 expression using the keyword super. It evaluates to a parent object.
super(arg1, arg2); // calls the parent constructor super.meth(arg1, arg2); // calls the parent object method
Spread operator
Spread operator named also rest operator allows an iterable to expand in place where multiple arguments (for function calls) or multiple elements (for array literals) are expected.
Function call
function sum(a, b, c) {
return a + b + c;
}
var items = [2, 3, 4];
var result = sum(...items); // 9
Array literal
var motorVehicles = ['car', 'motorcycle', 'truck']; var railedVehicles = ['train', 'tram']; var vehicles = ['bicke', ...motorVehicles, 'airplane', 'ship', ...railedVehicles]; console.log(vehicles); // [ "bicke", "car", "motorcycle", "truck", "airplane", "ship", "train", "tram" ]
Other expressions
There are many other expressions, such as arithmetic, comparison, logical. These are covered in the chapter on operators.
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