coercion-gotchas.md

false == "0" // true
// false gets coerced to 0
// "0" gets coerced to 0
// 0 == 0 is true 

false == 0 // true
// false gets coerced to 0
// 0 == 0 is true

false == "" // true
// false gets coerced to 0
// "" gets coerced to a number, 0
// 0 == 0 is true

false == [] // true
// false gets coerced to 0
// [] is an object so ToPrimitive would be called on it
// valueOf() would be consulted to find a primitive
// [].valueOf() would result in [], not a primitive value
// so [].toString() would be called, resulting in ""
// "" would then get coerced to a number, 0
// 0 == 0 is true

"" == 0 // true
// "" gets coerced to a number, 0
// 0 == 0 is true

"" == [] // true
// [] is an object so ToPrimitive would be called on it
// valueOf() would be consulted to find a primitive
// [].valueOf() would result in [], not a primitive value
// so [].toString() would be called, resulting in ""
// "" == "" is true

0 == [] // true
// [] is an object so ToPrimitive would be called on it
// valueOf() would be consulted to find a primitive
// [].valueOf() would result in [], not a primitive value
// so [].toString() would be called, resulting in ""
// "" would then get coerced to a number, 0
// 0 == 0 is true

Sources

• http://www.ecma-international.org/ecma-262/5.1/#sec-11.9.3
• http://www.ecma-international.org/ecma-262/5.1/#sec-9.1
• http://www.ecma-international.org/ecma-262/5.1/#sec-8.12.8
• https://github.com/getify/You-Dont-Know-JS/blob/master/types%20%26%20grammar/ch4.md#chapter-4-coercion