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The ECMAScript language draws heavily on the syntax of C and C languages (Java and Perl). So developers who are familiar with these languages will feel relaxed when they accept ECMAScript's more liberal syntax.
Case sensitive
The first concept to understand is that everything in ECMAScript (variables, functions, operators) are case-sensitive. This means that test and test represent two different variables, and the function name cannot be used typeof because it is a keyword. And typeof can be a valid function name entirely.
Identifier
The so-called identifiers are variables, functions, names of properties, or parameters of functions. The specification for identifiers is:
The first character must be a letter, an underscore, or a dollar sign;
Other characters can be letters, underscores, dollar signs, or numbers.
By convention, the ECMAScript identifier is in hump case format, that is, the first letter is lowercase. But there is no compulsion to use this approach.
Comments
One-line comment is two double slash//
Multi-line annotations are
/*
*/
Strict mode
Strict mode can be used with ' use strict '
Statement
The statement in ECMAScript is terminated with a semicolon, although the semicolon at the end of the statement is not required, we recommend that you do not omit him at any time.
Variable
ECMAScript variables are loosely typed, and so-called loose types can be used to hold any type of data. In other words, each variable is a placeholder for a saved value. When you define a variable, you also declare it with VAR, followed by the variable name. Such as
VAR message
This line of code defines a variable named message that can be used to hold variable values (such as uninitialized variables, which hold a special value undefined)
<!Doctype HTML><HTML><Head><title>Variable</title><MetaCharSet= "Utf-8" /></Head><Body> <Script> varmessage; alert (message); </Script></Div></Body></HTML>
Using the var operator to define a variable becomes a local variable in the scope that defines the variable. That is, if you define a variable with var in the function, the variable will be destroyed after the function exits.
<!Doctype HTML><HTML><Head><title>Variable</title><MetaCharSet= "Utf-8" /></Head><Body> <Script> functionTest () {varmessage; alert (message);//Eject undefined} test (); alert (message);//Error </Script></Div></Body></HTML>
The variable message is defined in the function with Var, when the function is called, and the variable is created and assigned a value. After this, the variables are destroyed immediately. So the next line of code in the example above will be an error. However, when declaring a variable, you can omit Var so that it becomes a global variable without an error. So as soon as the function is called, the variable is defined and can be called anywhere outside the function.
<!Doctype HTML><HTML><Head><title>Variable</title><MetaCharSet= "Utf-8" /></Head><Body> <Script> functionTest () {message= 'Hi'; alert (message); } test (); //Hialert (message);//Hi </Script></Div></Body></HTML>
When defining multiple variables, you can separate them with commas
<!Doctype HTML><HTML><Head><title>Variable</title><MetaCharSet= "Utf-8" /></Head><Body> <Script> functionTest () {varmessage='Hi', found= false, Age= in; alert (message); alert (age); } test (); //Hi </Script></Div></Body></HTML>
There are 5 basic types in ECMAScript: Undefined, null, Boolean, number, string, and a complex data type that object,object essentially consists of a set of unordered name-value pairs. ECMAScript does not support the creation of a mechanism for custom types, and all values are one of these 6 data types.
typeof operator
Since ECMAScript is loosely typed, there is a need for a means to detect the data type of a given variable. typeof is the operator responsible for providing this information. Using the typeof operator on a value may return one of the following strings
Undefined if this value is undefined
Boolean If this value is a Boolean value
String if this value is
Number if this value is a numeric type
Object If this value is a target or null
function if this value is
<!Doctype HTML><HTML><Head><title>Variable</title><MetaCharSet= "Utf-8" /></Head><Body> <Script> varmessage="some thing"; Alert (typeofmessage); Alert (typeof(message)); Alert (typeof the); </Script></Div></Body></HTML>
The above example shows that the operands of the typeof operator can be either a variable or a numeric literal. Note that TypeOf is an operator instead of a function, so the parentheses in the example, although it can be used, are not required.
JavaScript variable data type typeof
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