[Eloquent JS] Ch1. Values, Types, & Operators

1. Unary Operators

• Operators that take only one value (typeof, !).
• Minus operator can be both binary and unary.

console.log(typeof 4.5);
// -> number

console.log(-(10 - 2));
// -> -8

2. Comparison

• Strings are ordered alphabetically.
• Uppercase letters are always “less” than lowercase letters 'Z' < 'a'.
• Nan is the only value is JS that is not equal to itself.

console.log("Aardvark" < "Zoroaster");
// → true

console.log(NaN == NaN);
// → false

3. Empty Values

• null and undefined are themselves values, but carry no information.
• Treat them mostly interchangeable.

console.log(null == undefined);

4. Automatic Type Conversion

• JS automatically converts the value to the type it needs, when an operator is applied to the “wrong” type of value (type coercion).
• When a value does not map to a number in an obvious way, it converts to NaN.

console.log(8 * null);
// → 0
console.log("5" - 1);
// → 4
console.log("5" + 1);
// → 51
console.log("five" * 2);
// → NaN
console.log(false == 0);
// → true
console.log("" == 0);
// → true

• null and undefined only produces true, when both sides are null or undefined
• We can use this behavior to check whether a value has a real value or not.

console.log(null == undefined);
// → true
console.log(null == 0);
// → false

• If you do not want type conversions, you can use the strict equality operator: === and !==.
• Checks whether the values are precisely equal to one another.
• Use strict equality operator for most of the time.

console.log("" === false);
// → false

5. Short Circuiting of Logical Operators

• || and && can be used to handle values of different types.
• || will return the value to its left when it can be converted to true, otherwise false.
• Useful to check whether a value is empty, by placing the replacement value on the right.

console.log(null || "user");
// → user
console.log("Agnes" || "user");
// → Agnes

• && will return the value to its right when it can be converted to true, otherwise false.

console.log(null && "user");
// → null
console.log("Agnes" && "user");
// → user

• Part on their right will only be evaluated when necessary (short-circuit evaluation).
• true || x will only check the left part since it is true
• false && x will only check the right part since it is false