Why the following JavaScript code is not valid?

Question

When the following code is executed in chrome console

let a, b, c;
b = 2;
a = b+1 = c = 2+3;

It says an error 'Invalid left-hand side assignment' for the last statement. But when we execute the below-given code it accepts it and does not show any error.

let a, b, c;
a = b = c = 2+3;

assignment '=' is an operator so according to operator precedence in javascript, it should work fine. what do you guys think, what is the problem?

In the first case it is because you are doing b+1 = c, where c cannot be assigned to b+1. — Prashanth KR
– Prashanth KR, Commented Mar 12, 2018 at 17:29
c can be equat to 2+3, but b+1 cannot be equal to c. In other words a variable can take on a value, but a value can't take on a variable. That would be like saying 6 = 19. — Scott Marcus
– Scott Marcus, Commented Mar 12, 2018 at 17:31

Garret · Accepted Answer · 2018-03-12 17:30:43Z

1

for the first code you would need to do

let a,b,c;
b=2;
a=b+1;
c=5;

doing

a=b=c=2+3

works because you arent altering a value to the left of the last equal

answered Mar 12, 2018 at 17:30

Garret

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2 Comments

Abhishek Agrawal Over a year ago

it means we cannot have any operations on the left-hand side of the assignment operator. for example a +1 = 5 is not valid.

Garret Over a year ago

correct, if your trying to check if a+1 = 5 you would have to do 'a+1===5' as that would make it a Boolean

ykaner · Accepted Answer · 2018-03-12 17:40:19Z

1

the = operator calculate first the right side and require the left side to be lvalue.

In your second example for every assignment operator the left side is always a variable, so it works fine. but in the first example:

let a, b, c;
b = 2;
a = b+1 = c = 2+3;

you trying to assign 5 into b+1, its simply not a variable.

answered Mar 12, 2018 at 17:40

ykaner

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Comments

Jonas Wilms · Accepted Answer · 2018-03-12 17:56:28Z

0

You just need some parenthesis to make it clear what you mean:

 a = 1 + (b = c = 2+3);

answered Mar 12, 2018 at 17:56

Jonas Wilms

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