Chai syntax expect ([2]) .to.be.grammar ([2]) Why failed?

has been learning chai"s expect assertion library recently;
Why expect ([2]). To.be.equal ([2]) failed.
but expect ([2]). To.be.eql ([2]) succeeded.

Jun.04,2021
MySQL Query : SELECT * FROM `codeshelper`.`v9_news` WHERE status=99 AND catid='6' ORDER BY rand() LIMIT 5
MySQL Error : Disk full (/tmp/#sql-temptable-64f5-1e5f95a-44577.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
MySQL Errno : 1021
Message : Disk full (/tmp/#sql-temptable-64f5-1e5f95a-44577.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
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