Ternary conditions to find expmod?

I am reading SICP in JS about a non-terminating example of ternary conditions:

function is_even(n) {
    return n % 2 === 0;
}

function expmod(base, exp, m) {
    const half_exp = expmod(base, exp / 2, m);
    return exp === 0 ? 1
           : is_even(exp) ? half_exp * half_exp % m
           : base * expmod(base, exp - 1, m) % m;
}

console.log(expmod(4, 3, 5))

It explains that:

This would make the function not just inefficient, but actually non-terminating! The problem is that the constant declaration appears outside the conditional expression, which means that it is executed even when the base case exp === 0 is met.

I just cannot get its idea, when exp === 0, it terminate with 1 but why half_exp executed?

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