<ul><li>In the early days, var keyword is used usually to define a variable, but it would bring some troubles, such as variable obfuscation and memory leaks about variable.</li><li>Variables defined by var are declared at the top of their scope in advance and assigned the value underfined regardless of where they are declared.</li><li>After ES6, let and const keyword appeared while block scope was introduced. Block scope is differentiated by {  }, so if, for, while, etc all have their own block scope, but must use let, const keyword declared variables and var hasn't block scope yet.</li><li>At the same time, let, const haven't also hoisted-variable as var do.</li></ul>

Javascript - difference between var, let, and const

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