Currently going through Eloquent Javascript: The Annotated Version

These are my notes on chapter 1.

Values, Types, and Operators

  • Fractional calculations are not as precise as whole number calculations.

  • Straightforward String escape characters like

    "Hello,\n Testing \\, One\n\t, Two \"Three\""

    Becomes

      Hello,
  Testing \ One  
      Two "Three"

  • concatenation is done via

      "se" + "nd" + " " + "flow" + "ers"

    Becomes

      "send flowers"
  • unary and binary operators.
    • unary operators only take one value.
    • binary operators take two values.
    • ternary operators take three values.

  • typeof operator gives a string value defining the type of the given value

      console.log(typeof 4.5)
  // -> number
  • boolean types are defined as standard true/false

  • standard comparison operator

      console.log(3 == 5)
  // -> false
  console.log("family" = "family")
  // -> true
  • Standard ||, &&, and ! logical operators that equal, and not respectively.

  • tenary/conditional operator: Takes three values. and outputs the left or right value depending on whether or not the value left of the question mark is true or false.

      console.log(true ? 1 : 2);
  // → 1
  // left value is outputted because the value is true
  console.log(false ? 1 : 2);
  // → 2
  // right value is outputted because the value is false.
  console.log((100 > 1) ? "true" : "false")
  // -> "true"
  // true is output because 100 > 1 statement is true

Basically its a one line statement for an if else statement. where the if is left of : and else is right of :

  • Undefined values are null and undefined

Oh shit Javascript, what are you doing?

Apparently there is a thing called type coercion that allows things like

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

To happen. Thanks Javascript.

Basically Javascript converts values to types via its own rules unless it can’t. In which case it converts the values to NaN.

Additionally, If you try to do arithmetic operations on NaN it will keep producing NaN so if NaN is an error that you get then checking for accidental type conversions might solve the problem.

  • You can test if a value is a real value or not by comparing it to null.

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

  • 0, NaN and ("") count as false according to Javascript while everything else is true

Important Note about type conversions

=== comparison checks whether or not two values are precisely equal to each other and should be used defensively to prevent accidental type conversions.

	console.log(false === 0)
	// -> false

As it should

  • || and && operators try to convert the left side of comparisons to a boolean value if possible if both values are not boolean values themselves.

    • || will output the left value in it’s original type in a comparison if it can be converted to true but otherwise outputs the right value. console.log(null || “user”) // -> “user” console.log(“salt” || “mines”) // -> “salt”

    • && will do the opposite and give the right value if convertable but will otherwise output the left if not. console.log(“salty” && “bet”) // -> “bet” console.log(null && “aptitude”) // -> null

  • Short-circuit evaluation: Additional notes to || and && are that they will not evaluate expressions if they can get their first choice value as something valid i.e. true || X will output true and ignore X while false && X will output false