Học JavaScript qua ví dụ part 9

Hằng số Thời tiết và tâm trạng đang biến; thời gian là không đổi, và như vậy là tốc độ ánh sáng, giữa đêm, PI, và e. Trong chương trình, liên tục là một loại đặc biệt giữ chỗ với một giá trị không thể thay đổi trong quá trình thực hiện chương trình. Nhiều ngôn ngữ lập trình sử dụng một cú pháp đặc biệt để định nghĩa một hằng số để phân biệt với một biến. | Constants 67 EXAMPLE continued body html Output 3 25 cats 4 almost 25 5 29 6 510 years 7 30 dogs 8 dogs255 EXPLANATION 1 Variable x is assigned a number. 2 Variable y is assigned the string 510years. If the operator is used it could mean the concatenation of two strings or addition of two numbers. JavaScript looks at both of the operands. If one is a string and one is a number the number is converted to a string and the two strings are joined together as one string so in this example the resulting string is 510years. If one operand were 5 and the other 10 addition would be performed resulting in 15. 3 A number is concatenated with a string. The number 25 is converted to a string and concatenated to cats resulting in 25 cats. Note that the write method can also use commas to separate its arguments. In these examples the br tag is not concatenated to the string. It is sent to the write method and appended. 4 This time a string is concatenated with a number resulting in the string almost 25. 5 When the operands on either side of the sign are numbers addition is performed. 6 The value of y a string is displayed. 7 The operators works from left to right. Because x and y are both numbers addition is performed 25 5. 30 is concatenated with the string dogs . 8 Because the works from left to right this time the first operand is a string being concatenated to a number the number is converted to string dogs25 and concatenated with string 5. Constants The weather and moods are variable time is constant and so are the speed of light midnight PI and e. In programming a constant is a special kind of placeholder with a value that cannot be changed during program execution. Many programming languages use a special syntax to define a constant to distinguish it from a variable. JavaScript declares constants with the const type which replaces var and the name of the constant is in From the Library of 68 Chapter 3 The Building Blocks Data Types Literals and .