Language Definitions for JSX files.
Emmet's default is to not support JS files. So you will need to add a keyboard shortcut to tab complete in JSX files.
open up Preferences > Key Bindings - user and add this entry:
{
"keys": ["tab"],
"command": "expand_abbreviation_by_tab",
"context": [
{
"operand": "source.js.jsx",
"operator": "equal",
"match_all": true,
"key": "selector"
},
{
"key": "selection_empty",
"operator": "equal",
"operand": true,
"match_all": true
}
]
}