telemachus ~$ coffee
coffee> @a = 1
1
coffee> @a ?= 4
1
coffee> @a
1
coffee> a = 3
3
coffee> a
3
coffee> @a
3
coffee> this.a
3
coffee> a ?= 3
Error: In repl, the variable "a" can't be assigned with ?= because it has not been defined.
    at Assign.compileConditional (/Users/telemachus/local/node-v0.6.14/lib/node_modules/coffee-script/lib/coffee-script/nodes.js:1635:15)
    at Assign.compileNode (/Users/telemachus/local/node-v0.6.14/lib/node_modules/coffee-script/lib/coffee-script/nodes.js:1497:23)
    at Assign.compile (/Users/telemachus/local/node-v0.6.14/lib/node_modules/coffee-script/lib/coffee-script/nodes.js:48:21)
    at Block.compileNode (/Users/telemachus/local/node-v0.6.14/lib/node_modules/coffee-script/lib/coffee-script/nodes.js:320:27)
    at Block.<anonymous> (/Users/telemachus/local/node-v0.6.14/lib/node_modules/coffee-script/lib/coffee-script/nodes.js:48:21)
    at Block.compile (/Users/telemachus/local/node-v0.6.14/lib/node_modules/coffee-script/lib/coffee-script/nodes.js:291:40)
    at Parens.compileNode (/Users/telemachus/local/node-v0.6.14/lib/node_modules/coffee-script/lib/coffee-script/nodes.js:2500:19)
    at Parens.compile (/Users/telemachus/local/node-v0.6.14/lib/node_modules/coffee-script/lib/coffee-script/nodes.js:48:21)
    at Value.compileNode (/Users/telemachus/local/node-v0.6.14/lib/node_modules/coffee-script/lib/coffee-script/nodes.js:647:24)
    at Value.compile (/Users/telemachus/local/node-v0.6.14/lib/node_modules/coffee-script/lib/coffee-script/nodes.js:48:21)