Uses the short circuit property of&&.

inA && B

means that ifAis false, then the entire expression is false and there is no need to evaluateB.
IfAis true, then evaluateBto judge

So the above code means

Ifelis true, then executefn.call(el, e, el);

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||also has similar properties:

If the lvalue is true, the following does not need to be evaluated.

For example, used to specify the default value

function test(a){ a = a || '默认值'; return a; } test(); // => '默认值' test('wow'); // => 'wow'

If el exists, the following function will be called, otherwise it will not be called

// 如果el为空会报错，所以加个判断 if(el){ fn.call(el, e, el) }

a && b is equivalent to
if (a){

b

}