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undefinedVariables

Reports using variables that are not defined.

✅ This rule is included in the ts javascript presets.

Variables must be declared before they can be used in JavaScript. Using undefined variables will cause a ReferenceError at runtime. This rule helps catch typos and missing declarations early in development.

Examples

Section titled “Examples”
53consoAB0l53(undefinedVar<);d ivclassNa="meco"><);d>AEfuncti5<3 sp ansty="le--0:#82AAFF;--1:#3B61">B0calcula11<()sp ansty="le--0:#D6DEEB;--1:#403F">53< {d ivclassNa="meec-li">nt< sp ansty="le--0:#C792EA;--1:#8844">AEretu53 resul5<3}d ivclassNa="meco"><}d>AEconsB0valAE <= sp ansty="le--0:#D6DEEB;--1:#403F">53firAE <+ sp ansty="le--0:#D6DEEB;--1:#403F">53secon
44AEco61B0v44A0<9825 spans="tyle--0:#D6DEEB;--1:#40">33F53con4<4AE. spans="tyle--0:#82AAFF;--1:#3B">61B<0log spans="tyle--0:#D6DEEB;--1:#40">3F53(val<;di vdivclass="Nameexpressive-">44AEfunc361B0calcu1<111( spans="tyle--0:#D7DBE0;--1:#40">3F53i6361B0nu1<111) spans="tyle--0:#D6DEEB;--1:#40">3F<53 {divclass="Nameec-">de44AEco61B0re44A3F53i44A0<9822 spans="tyle--0:#D6DEEB;--1:#40">3de44AEre3F53 res3<}di vdivclass="Nameexpressive-">44AEco61B0f44A0<9821 spans="tyle--0:#D6DEEB;--1:#40">344AEco61B0se44A0<9822 spans="tyle--0:#D6DEEB;--1:#40">344AEco61B0v44A3F53f44A3F53sec

Options

Section titled “Options”

This rule is not configurable.

When Not To Use It

Section titled “When Not To Use It”

This rule can be safely disabled if you’re using TypeScript’s type checking, which will catch undefined variables through its own error reporting.

Further Reading

Section titled “Further Reading”

Equivalents in Other Linters

Section titled “Equivalents in Other Linters”