Why does this IFS variable assignment not affect the cmd it precedes?

Question

Consider this bash session:

set -- 1 2 3
echo "$*"          # 1 2 3  (as expected)
IFS=a echo "$*"    # 1 2 3  (why not "1a2a3"?)
IFS=a; echo "$*"   # 1a2a3a (as expected)

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Why does the "assignment before command" syntax not work to set IFS to a different value for the duration of the single command? It appears you must change the value of IFS first with a separate command.

Answer 1

Because IFS affects the parsing (tokenization, word-splitting) of a command, so it must be set before that command is parsed. This is why IFS=a echo "$*" can only use the original IFS, not a.

It's a somewhat similar case to FOO=bar echo $FOO not echoing bar. $FOO is substituted (empty) and then the command, with it's variable assignment is executed.