Преобразование строк JavaScript и преобразование чисел
Я обнаружил, что добавление в конструктор - хороший обходной путь для новых объектов по умолчанию для значения:
public EntityName(){
this.fieldToDefault = default;
}
6 ответов
Вы хотите познакомиться с parseInt () и toString () .
И полезно в вашем наборе инструментов будет посмотреть на переменная, чтобы узнать, какой это тип - typeof :
<script type="text/javascript">
/**
* print out the value and the type of the variable passed in
*/
function printWithType(val) {
document.write('<pre>');
document.write(val);
document.write(' ');
document.writeln(typeof val);
document.write('</pre>');
}
var a = "1", b = "2", c = "3", result;
// Step (1) Concatenate "1", "2", "3" into "123"
// - concatenation operator is just "+", as long
// as all the items are strings, this works
result = a + b + c;
printWithType(result); //123 string
// - If they were not strings you could do
result = a.toString() + b.toString() + c.toString();
printWithType(result); // 123 string
// Step (2) Convert "123" into 123
result = parseInt(result,10);
printWithType(result); // 123 number
// Step (3) Add 123 + 100 = 223
result = result + 100;
printWithType(result); // 223 number
// Step (4) Convert 223 into "223"
result = result.toString(); //
printWithType(result); // 223 string
// If you concatenate a number with a
// blank string, you get a string
result = result + "";
printWithType(result); //223 string
</script>
Step (1) Concatenate "1", "2", "3" into "123"
"1" + "2" + "3"
or
["1", "2", "3"].join("")
The join method concatenates the items of an array into a string, putting the specified delimiter between items. In this case, the "delimiter" is an empty string ("").
Step (2) Convert "123" into 123
parseInt("123")
Prior to ECMAScript 5, it was necessary to pass the radix for base 10: parseInt("123", 10)
Step (3) Add 123 + 100 = 223
123 + 100
Step (4) Covert 223 into "223"
(223).toString()
Put It All Togther
(parseInt("1" + "2" + "3") + 100).toString()
or
(parseInt(["1", "2", "3"].join("")) + 100).toString()
You can do it like this:
// step 1
var one = "1" + "2" + "3"; // string value "123"
// step 2
var two = parseInt(one); // integer value 123
// step 3
var three = 123 + 100; // integer value 223
// step 4
var four = three.toString(); // string value "223"
These questions come up all the time due to JavaScript's typing system. People think they are getting a number when they're getting the string of a number.
Here are some things you might see that take advantage of the way JavaScript deals with strings and numbers. Personally, I wish JavaScript had used some symbol other than + for string concatenation.
Step (1) Concatenate "1", "2", "3" into "123"
result = "1" + "2" + "3";
Step (2) Convert "123" into 123
result = +"123";
Step (3) Add 123 + 100 = 223
result = 123 + 100;
Step (4) Convert 223 into "223"
result = "" + 223;
If you know WHY these work, you're less likely to get into trouble with JavaScript expressions.
Чтобы преобразовать строку в число, вычтите 0. Чтобы преобразовать число в строку, добавьте "" (пустую строку).
5 + 1 даст вам 6
(5 + "") + 1 даст вам "51"
("5 "- 0) + 1 даст вам 6
parseInt is misfeatured like scanf:
parseInt("12 monkeys", 10) is a number with value '12'
+"12 monkeys" is a number with value 'NaN'
Number("12 monkeys") is a number with value 'NaN'