Actually, if you read the error message properly, case 1 and case 2 throw different errors.
Case a.x.y:
Cannot set property ‘y’ of undefined
Case a.x.y.z:
Cannot read property ‘y’ of undefined
I guess it’s best to describe it by step-by-step execution in easy English.
Case 1
// 1. Declare variable `a`
// 2. Define variable `a` as {}
var a = {}
// 1. Declare variable `b`
// 2. Define variable `b` as {}
var b = {}
try {
/**
* 1. Read `a`, gets {}
* 2. Read `a.x`, gets undefined
* 3. Read `b`, gets {}
* 4. Set `b.z` to 1, returns 1
* 5. Set `a.x.y` to return value of `b.z = 1`
* 6. Throws "Cannot **set** property 'y' of undefined"
*/
a.x.y = b.z = 1
} catch(e){
console.error(e.message)
} finally {
console.log(b.z)
}Case 2
// 1. Declare variable `a`
// 2. Define variable `a` as {}
var a = {}
// 1. Declare variable `b`
// 2. Define variable `b` as {}
var b = {}
try {
/**
* 1. Read `a`, gets {}
* 2. Read `a.x`, gets undefined
* 3. Read `a.x.y`, throws "Cannot **read** property 'y' of undefined".
*/
a.x.y.z = b.z = 1
} catch(e){
console.error(e.message)
} finally {
console.log(b.z)
}In comments, Solomon Tam found this ECMA documentation about assignment operation.