What is claimed is:1. A handheld electronic device comprising:a case including a bottom, a cover connected to the bottom, at least one rib, an air inlet and a heat outlet, wherein the bottom and the cover delimit an interior of the case, wherein the at least one rib is disposed in the interior of the case and delimits a heat chamber in the interior of the case, and wherein the air inlet and the heat outlet intercommunicate with an outside of the case;an electronic unit including a heat source located in the heat chamber; anda cooling fan located in the heat chamber and including an air entrance and an air exit, wherein the air entrance intercommunicates with the air inlet of the case, and wherein the air exit intercommunicates with the heat outlet of the case.2. The handheld electronic device as claimed in claim 1, wherein the heat source and the cooling fan are coupled with the bottom, and wherein the at least one rib is provided on the cover.3. The handheld electronic device as claimed in claim 2, wherein the cover includes a lateral wall connected to an edge of a back plate, and wherein the air inlet and the heat outlet are located on two opposite sides of the lateral wall.4. The handheld electronic device as claimed in claim 3, wherein the air entrance of the cooling fan faces the air inlet of the case, wherein the air exit of the cooling fan faces the heat outlet of the case, and wherein the heat source is located between the air inlet of the case and the air entrance of the cooling fan.5. The handheld electronic device as claimed in claim 3, wherein the air entrance of the cooling fan faces the air inlet of the case, wherein the air exit of the cooling fan faces the heat outlet of the case, and wherein the heat source is located between the air exit of the cooling fan and the heat outlet of the case.6. The handheld electronic device as claimed in claim 3, wherein the at least one rib includes two ribs, and wherein each of the two ribs includes two ends respectively connected to the two opposite sides of the lateral wall.7. The handheld electronic device as claimed in claim 3, wherein a bottom edge of the air entrance is spaced from the back plate at a maximum height, and wherein each of the at least one rib is spaced from the back plate at a height larger than or equal to the maximum height.8. The handheld electronic device as claimed in claim 2, wherein the cover includes a lateral wall connected to an edge of a back plate, wherein the air inlet is located on the back plate, and wherein the heat outlet is located on the lateral wall.9. The handheld electronic device as claimed in claim 8, wherein the air entrance of the cooling fan faces the air inlet of the case and the air exit of the cooling fan faces the heat outlet of the case, and wherein the heat source is located between the air exit of the cooling fan and the heat outlet of the case.10. The handheld electronic device as claimed in claim 8, wherein the heat source is located between the air inlet of the case and the air entrance of the cooling fan, and wherein the air exit of the cooling fan faces the heat outlet of the case.11. The handheld electronic device as claimed in claim 8, wherein the at least one rib includes two ribs, and wherein each of the two ribs includes two ends respectively connected to two opposite sides of the lateral wall.12. The handheld electronic device as claimed in claim 8, wherein the at least one rib includes a single rib having two ends connected to a same side of the lateral wall.13. The handheld electronic device as claimed in claim 8, wherein the at least one rib includes a single rib having two ends respectively connected to two opposite sides of the lateral wall.14. The handheld electronic device as claimed in claim 8, wherein the at least one rib includes a plurality of ribs connected to each other in sequence, wherein a first one of the plurality of ribs has a free end connected to the lateral wall, and wherein a last one of the plurality of ribs has a free end connected to the lateral wall.15. The handheld electronic device as claimed in claim 2, wherein the at least one rib includes two ribs respectively connected to the bottom and the cover of the case.16. The handheld electronic device as claimed in claim 15, wherein one of the two ribs which is connected to the cover includes an extension portion, and wherein the extension portion is connected to a lateral face of said rib at a free end of said rib.17. The handheld electronic device as claimed in claim 2, wherein the cover includes a lateral wall connected to an edge of a back plate, wherein the air inlet is located on the lateral wall, and wherein the heat outlet is located on the cover.18. The handheld electronic device as claimed in claim 17, wherein the cooling fan is spaced from the bottom of the case at a gap, wherein the air entrance of the cooling fan faces the bottom of the case, wherein the air exit of the cooling fan faces the heat outlet of the case, and wherein the heat source is located between the air inlet of the case and the cooling fan.19. The handheld electronic device as claimed in claim 1, wherein each of the at least one rib has a free end connected to a buffering portion.