SICP(1.2.5) + https://blog.csdn.net/aimingoo/article/details/51136511

In general, statements are not evaluated but executed; they do not produce a value but instead make some change.

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 >>> x = 3
does not return a value nor evaluate a function on some arguments, since the purpose of assignment is instead to bind a name to a value.

JS当中:

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> a = 1
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