Lesson 04 · Basics

Operators and equality

An operator is a symbol that takes values and makes a new value. Follow a seven-line script as it stores a number and a string, works out 7 + 3 * 2 (multiplication first), joins text with +, and then compares 7 with "7" twice: == converts the string to a number and says true, while === never converts and says false.

9 min9 steps5 questionsBeginner

Fig 4 · Interactive

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5 quick questions

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01What is the value of 2 + 3 * 4?

* goes before +, so 3 * 4 = 12 is worked out first, then 2 + 12 = 14. Write (2 + 3) * 4 if you want 20.

02What does "5" + 2 give?

When either side of + is a string, + joins text. "5" + 2 becomes "5" + "2", which is the string "52".

03What is 7 == "7", and why?

Loose equality == converts values of different types before comparing. The string "7" becomes the number 7, and 7 equals 7.

04What is 7 === "7"?

Strict equality === never converts. A number and a string are different types, so they are not strictly equal.

05In let x = 10;, what does the = do?

A single = is the assignment operator: it stores the value on its right in the variable on its left. == and === are the ones that compare.

Key terms

Glossary

Operator: A symbol such as +, *, === or && that takes one or more values and makes a new value.
Operand: A value an operator works on. In 7 + 6, the operands are 7 and 6.
Assignment (=): Stores the value on the right in the variable on the left. It does not compare.
Precedence: The order operators are worked out in when there are several, for example * before +. Brackets change it.
Concatenation: Joining strings end to end. In JavaScript, + concatenates when either side is a string.
Loose equality (==): Compares two values after converting them to the same type when their types differ.
Strict equality (===): Compares two values without converting them: values of different types are never equal. !== is its opposite.
Logical AND (&&): Gives true only when both sides are true (strictly, it returns one of its operands).

Fine print

Notes and sources

The full rules for == (IsLooselyEqual in the ECMAScript specification) have several cases. A string compared with a number is turned into a number first, and null == undefined is true. === (IsStrictlyEqual) never converts: values of different types are never equal.
If either side of + is a string, + joins text; otherwise it adds numbers. The other arithmetic operators, - * / and % (remainder), always work on numbers, so "7" - 3 is 4.
&& really returns one of its operands: the left one if it is falsy, otherwise the right one. With true and false on both sides, that is the same as "true only when both are true".
One surprise even for ===: NaN === NaN is false. Use Number.isNaN() to check for NaN.
The figure draws comparing as its own station so you can see it. In reality the JavaScript engine does every operator, including the comparisons.
The script is real. Paste it into the browser console, or save it as ops.js and run node ops.js: it prints true, false, true.

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