Lecture
There are several basic data types in JavaScript.
number :
var n = 123; |
n = 12.345; |
There are special numerical values Infinity (infinity) and NaN (computation error). They also belong to the type "number".
For example, Infinity infinity is obtained by dividing by zero:
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alert( 1 / 0 ); // Infinity |
The NaN calculation error will be the result of an incorrect mathematical operation, for example:
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alert( "нечисло" * 2 ); // NaN, ошибка |
var str = "Мама мыла раму" ; |
str = 'Одинарные кавычки тоже подойдут' ; |
Some programming languages have a special data type for one character. For example, in C, this is char . In JavaScript, there is only the type "string" string . Which, I must say, is quite convenient ..
boolean . It has only two values - true (true) and false (false).
Typically, this type is used to store a yes / no value, for example:
var checked = true ; // поле формы помечено галочкой |
checked = false ; // поле формы не содержит галочки |
null is a special value. It makes sense of “nothing.” The null value does not belong to any of the types above, but forms its own separate type consisting of a single null value:
var age = null ; |
null not a “reference to a non-existent object” or “null pointer”, as in some other languages. This is just a special meaning that makes sense of “nothing” or “value unknown”.
In particular, the code above says that age unknown.
undefined is a special value that, like null , forms its own type. It makes sense "not assigned".
If a variable is declared, but nothing is written to it, then its value is exactly undefined :
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var u; |
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alert(u); // выведет "undefined" |
undefined and explicitly, although this is rarely done:
var x = 123; |
x = undefined ; |
In an explicit form, undefined usually not assigned, since it contradicts its meaning. To write to the "empty value" variable, use null .
The first 5 types are called "primitive" .
The sixth type stands apart: “objects” . It includes, for example, dates, it is used for data collections and for many other things. Later we will return to this type and consider its fundamental differences from primitives.
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