Automatic conversion
JavaScript is a weak type programming language. It will always try to execute the code, regardless of the given value's data types, even if it does not make sense. For example:
1console.log(1 + "1");111In this case, JavaScript will assume you are trying to perform a concatenation operation, and it will convert the number 1 into a string and then join the two strings together.
1console.log(5 * null);10And in this example, null will be converted into the number 0, and 5 times 0 gives 0.
Even though these examples technically work, you should never rely on automatic conversions, as they can lead to unexpected problems.
A better way to code is to convert the values involved in the operation explicitly to ensure they are always the intended types.
Convert to strings
For example, if you intend to perform string concatenation, then make sure all the values are converted into strings using the String() function.
1console.log(typeof String(null));
2console.log(typeof String(undefined));
3console.log(typeof String(true));
4console.log(typeof String(false));
5console.log(typeof String(123));
6console.log(typeof String("123"));1string
2string
3string
4string
5string
6stringThe Boolean and number types also have a built-in toString() method that can do the same thing. Except it doesn't work on the value directly. You must pass the value to a variable and then call the method through the variable name.
1let a = true;
2let b = false;
3let c = 123;
4let d = "123";
5
6console.log(typeof a.toString());
7console.log(typeof b.toString());
8console.log(typeof c.toString());
9console.log(typeof d.toString());1string
2string
3string
4stringThe String() method converts the value itself into a string, while the toString() method returns a converted result without altering the original value.
Convert to numbers
The Number() function can be used to convert other data types into numbers. The conversion obeys the following rules:
- Strings that contain only white spaces will be converted into 0.
1console.log(Number("\n\t "));10- Strings that can be converted into numbers without errors, meaning they contain only number digits or a sign in the front, will be converted into the corresponding number.
1console.log(Number("00001"));
2console.log(Number("12345"));
3console.log(Number("-12345"));
4console.log(Number("-0.001"));11
212345
3-12345
4-0.001- The leading and tailing white spaces will be automatically removed.
1console.log(Number("\t00001\t"));
2console.log(Number("12345\n"));11
212345- If an error is encountered during conversion, meaning one or more characters can not be converted, then the string will become
NaN.
1console.log(Number("000a01"));
2console.log(Number("123 45"));
3console.log(Number("-12-345"));
4console.log(Number("-0.00.1"));1NaN
2NaN
3NaN
4NaN- For Boolean values,
truewill become 1, andfalsewill become 0.
1console.log(Number(true));
2console.log(Number(false));11
20nullwill become 0, andundefinedwill becomeNaN. This is also one of the few differences betweennullandundefined. But again, in practice, you are unlikely to encounter circumstances where you need to differentiate the two explicitly.
1console.log(Number(null));
2console.log(Number(undefined));10
2NaNConvert to Boolean values
The Boolean() function is used to convert other data types into either true or false. As we've demonstrated before, all falsy values will become false:
1console.log(Boolean(null));
2console.log(Boolean(undefined));
3console.log(Boolean(0));
4console.log(Boolean(""));1false
2false
3false
4falseAnd all truthy values will become true:
1console.log(Boolean(10));
2console.log(Boolean(-10));
3console.log(Boolean(" "));
4console.log(Boolean("qwerty"));1true
2true
3true
4true