assignment in javascript and the var keyword

Per ECMA-262 § 12.2, a VariableStatement (that is, var identifier=value) explicitly returns nothing. Additionally, a VariableStatement is a Statement; Statements do not return values, and it is invalid to put a Statement somewhere an Expression would go.

For example, none of the following make sense because they put a Statement where you need value-yielding Expressions:

var a = var b;
function fn() { return var x; }

Per § 11.13.1, assignment to a variable (identifier=value) returns the assigned value.

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When you write var a = 1;, it declares a and initalizes its value to 1. Because this is a VariableStatement, it returns nothing, and the REPL prints undefined.

a=1 is an expression that assigns 1 to a. Since there is no a, JavaScript implicitly creates a global a in normal code (but would throw a ReferenceError in strict mode, since you're not allowed to implicitly create new globals in strict mode).

Regardless of whether or not a existed before, the expression still returns the assigned value, 1, so the REPL prints that.