During a recent interview, I was presented with this question about JavaScript evaluation order. My understanding is that in JavaScript, evaluation proceeds from left to right. So would 1 && 2 result in false? I came across another discussion where it was mentioned that 1 && 2 actually returns 2, and the AND operator returns the first falsy value. However, this explanation felt a bit unclear. Can someone provide more clarity on this topic?
The AND (&&) operator functions as follows:
It evaluates operands in a left-to-right order.
Each operand is converted to a boolean. If the result is false, the process stops and returns the original value of that operand.
If all operands have been evaluated (i.e., all were truthy), it returns the last operand.
In simpler terms, the AND operator returns the first falsy value encountered or the last value if none are found. When all values are truthy, the last value is returned.
Ref: https://javascript.info/logical-operators
1&&2 should be considered false, right?
Actually, in JavaScript, in a Boolean context, these values are treated as truthy. Only six specific values are recognized as falsy: false, 0, the empty string (e.g., '', ""), null, undefined, and NaN.
1 && 2 && 3
equivalent to
if (1) {
if (2) {
if (3) {
return 3
} else {
return 3
}
} else {
return 2
}
} else {
return 1
}
1 && 2 && 3 //equals 3
1 && 0 && 3 //equals 0
1 && false && 3 //equals false
false && 0 && 3 //equals false
1 && 0 && false //equals 0
In the case that the condition on the left side of && is false, the entire expression will be evaluated to the value on the left.
However, if the condition is true, then the expression evaluates to the value on the right.
This explains why 1&&2&&3 results in 3.
Learn more about logical AND operations where the last truthy value is returned if all values are truthy, otherwise the first falsy value encountered.
console.log(1 && 2 && 3); // 3
console.log(1 && 0 && null); // 0When using the logical operator "&&", all numbers except 0 are considered true. If both operands are true, the operator returns the second operand. For example, 1&&2 will return 2 and 2&&3 will return 3.
&& functions as the logical AND operator for expressions. The evaluation of individual expressions follows the standard rules, where the outcomes are not strictly 'true' or 'false', but rather considered 'truthy' or 'falsy' (referenced in section 12.13 of the ECMAScript standard).
Therefore, the JavaScript engine is required to assess all expressions until encountering a falsy value (the final outcome being this falsy value interpreted as false) or when no more expressions are present.
According to the standard, evaluations take place from left to right (section 12.13.3 of ECMAScript standard).
The definitions of Falsy and Truthy are:
false, 0, "", null, undefined, NaNThese definitions can be found in the explanation of the ToBoolean function within section 7.1.2 of the standard. Refer to MDN glossary for a concise overview (falsy, truthy)
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